feat: replace header dropdowns with collapsible sidebar tree

- Add Sidebar.tsx: project/session tree with inline rename, collapse/resize
- App.tsx: load all sessions at startup, sync selectedProject on session click
- Header.tsx: strip project/session UI, keep only right-side controls
- globals.css: add .main-layout, sidebar, item, and activity-dot styles
- Chat pane: move toggle button to left, use triangle icons matching sidebar

1 files changed, 318 insertions, 19 deletions

diff --git a/renderer/src/styles/globals.css b/renderer/src/styles/globals.css
index 20275ae..c463432 100644
--- a/renderer/src/styles/globals.css
+++ b/renderer/src/styles/globals.css
@@ -122,23 +122,6 @@ body {
   background: var(--bg-tertiary);
 }
 
-.session-rename-input {
-  padding: 5px 10px;
-  background: var(--bg-tertiary);
-  border: 1px solid var(--accent);
-  border-radius: 2px;
-  color: var(--text-primary);
-  font-size: 12px;
-  font-family: inherit;
-  min-width: 140px;
-  outline: none;
-  box-shadow: 0 0 0 2px rgba(96, 165, 250, 0.2);
-}
-
-html[data-theme="light"] .session-rename-input {
-  box-shadow: 0 0 0 2px rgba(37, 99, 235, 0.2);
-}
-
 /* Theme toggle */
 .theme-toggle {
   font-size: 11px;
@@ -167,6 +150,17 @@ html[data-theme="light"] .session-rename-input {
   border-radius: 2px;
   background: var(--bg-tertiary);
   color: var(--text-secondary);
+  border: none;
+  font-family: inherit;
+}
+
+/* Reachable phases rendered as <button> — add pointer + hover */
+button.phase-step {
+  cursor: pointer;
+}
+
+button.phase-step:hover {
+  filter: brightness(1.15);
 }
 
 .phase-step.active {
@@ -179,6 +173,12 @@ html[data-theme="light"] .session-rename-input {
   color: white;
 }
 
+/* Viewing a historical artifact: outline on the viewed badge */
+.phase-step.viewing {
+  outline: 2px solid var(--accent);
+  outline-offset: -2px;
+}
+
 /* ── Branch Badge ────────────────────────────────────────────── */
 .branch-badge {
   padding: 3px 10px;
@@ -206,13 +206,277 @@ html[data-theme="light"] .session-rename-input {
   color: white;
 }
 
-/* ── Main Content ─────────────────────────────────────────────── */
+/* ── Main Layout (sidebar + content row) ─────────────────────── */
+.main-layout {
+  flex: 1;
+  display: flex;
+  overflow: hidden;
+}
+
+/* ── Main Content (document + chat row) ──────────────────────── */
 .main-content {
   flex: 1;
   display: flex;
   overflow: hidden;
 }
 
+/* ── Sidebar ─────────────────────────────────────────────────── */
+.sidebar {
+  display: flex;
+  flex-direction: column;
+  background: var(--bg-secondary);
+  border-right: 1px solid var(--border);
+  flex-shrink: 0;
+  overflow: hidden;
+  min-width: 0;
+}
+
+.sidebar.collapsed {
+  width: 32px !important;
+  align-items: center;
+  padding-top: 8px;
+}
+
+.sidebar-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 8px 10px;
+  border-bottom: 1px solid var(--border);
+  flex-shrink: 0;
+}
+
+.sidebar-title {
+  font-size: 10px;
+  font-weight: 700;
+  letter-spacing: 0.12em;
+  text-transform: uppercase;
+  color: var(--text-secondary);
+  user-select: none;
+}
+
+.sidebar-header-actions {
+  display: flex;
+  gap: 4px;
+}
+
+.sidebar-action-btn,
+.sidebar-collapse-btn {
+  background: transparent;
+  border: none;
+  color: var(--text-secondary);
+  cursor: pointer;
+  font-size: 13px;
+  font-family: inherit;
+  padding: 2px 5px;
+  border-radius: 2px;
+  line-height: 1;
+  transition: background 0.1s, color 0.1s;
+}
+
+.sidebar-action-btn:hover,
+.sidebar-collapse-btn:hover {
+  background: var(--bg-tertiary);
+  color: var(--text-primary);
+}
+
+.sidebar-tree {
+  flex: 1;
+  overflow-y: auto;
+  padding: 4px 0;
+}
+
+/* ── Sidebar Resize Handle ───────────────────────────────────── */
+.sidebar-resize-handle {
+  width: 5px;
+  background: transparent;
+  cursor: col-resize;
+  flex-shrink: 0;
+  transition: background 0.15s;
+}
+
+.sidebar-resize-handle:hover {
+  background: var(--accent);
+}
+
+/* ── Project Items ───────────────────────────────────────────── */
+.project-item {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 5px 10px;
+  cursor: default;
+  user-select: none;
+  gap: 4px;
+}
+
+.project-item:hover {
+  background: var(--bg-tertiary);
+}
+
+.project-item.selected {
+  background: var(--bg-tertiary);
+}
+
+.project-name {
+  flex: 1;
+  font-size: 11px;
+  font-weight: 600;
+  letter-spacing: 0.04em;
+  text-transform: uppercase;
+  color: var(--text-primary);
+  cursor: pointer;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  min-width: 0;
+}
+
+.project-item.selected .project-name {
+  color: var(--accent);
+}
+
+/* ── Session Items ───────────────────────────────────────────── */
+.session-item {
+  display: flex;
+  align-items: center;
+  padding: 4px 10px 4px 22px;
+  gap: 4px;
+  user-select: none;
+}
+
+.session-item:hover {
+  background: var(--bg-tertiary);
+}
+
+.session-item.selected {
+  background: var(--bg-tertiary);
+  border-left: 2px solid var(--accent);
+  padding-left: 20px;
+}
+
+.session-name {
+  flex: 1;
+  font-size: 12px;
+  color: var(--text-secondary);
+  cursor: pointer;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  min-width: 0;
+  display: flex;
+  align-items: center;
+  gap: 5px;
+}
+
+.session-item.selected .session-name {
+  color: var(--text-primary);
+}
+
+/* ── Activity dot ────────────────────────────────────────────── */
+.session-activity-dot {
+  display: inline-block;
+  width: 6px;
+  height: 6px;
+  border-radius: 50%;
+  background: var(--accent);
+  flex-shrink: 0;
+  animation: activity-pulse 1.8s ease-in-out infinite;
+}
+
+@keyframes activity-pulse {
+  0%, 100% { opacity: 0.4; }
+  50%       { opacity: 1;   }
+}
+
+/* ── Shared item controls (shown on hover) ───────────────────── */
+.item-controls {
+  display: flex;
+  gap: 2px;
+  flex-shrink: 0;
+  opacity: 0;
+  transition: opacity 0.1s;
+}
+
+.project-item:hover .item-controls,
+.session-item:hover .item-controls {
+  opacity: 1;
+}
+
+.session-item.selected .item-controls {
+  opacity: 1;
+}
+
+.item-btn {
+  background: transparent;
+  border: none;
+  color: var(--text-secondary);
+  cursor: pointer;
+  font-size: 11px;
+  font-family: inherit;
+  padding: 1px 4px;
+  border-radius: 2px;
+  line-height: 1.4;
+  transition: background 0.1s, color 0.1s;
+}
+
+.item-btn:hover {
+  background: var(--bg-primary);
+  color: var(--text-primary);
+}
+
+.item-btn-danger:hover {
+  background: var(--danger);
+  color: white;
+}
+
+/* ── Sidebar empty states ────────────────────────────────────── */
+.sidebar-empty {
+  font-size: 11px;
+  color: var(--text-secondary);
+  font-style: italic;
+  padding: 10px 12px;
+  text-align: center;
+}
+
+.session-empty {
+  padding: 4px 10px 4px 22px;
+  text-align: left;
+}
+
+/* ── Sidebar scrollbar ───────────────────────────────────────── */
+.sidebar-tree::-webkit-scrollbar {
+  width: 4px;
+}
+
+.sidebar-tree::-webkit-scrollbar-track {
+  background: transparent;
+}
+
+.sidebar-tree::-webkit-scrollbar-thumb {
+  background: var(--bg-tertiary);
+  border-radius: 2px;
+}
+
+/* ── Session rename input (in sidebar) ───────────────────────── */
+.session-rename-input {
+  flex: 1;
+  padding: 2px 6px;
+  background: var(--bg-tertiary);
+  border: 1px solid var(--accent);
+  border-radius: 2px;
+  color: var(--text-primary);
+  font-size: 12px;
+  font-family: inherit;
+  min-width: 0;
+  outline: none;
+  box-shadow: 0 0 0 2px rgba(96, 165, 250, 0.2);
+}
+
+html[data-theme="light"] .session-rename-input {
+  box-shadow: 0 0 0 2px rgba(37, 99, 235, 0.2);
+}
+
 /* ── Document Pane ───────────────────────────────────────────── */
 .document-pane {
   flex: 1;
@@ -406,6 +670,11 @@ html[data-theme="light"] .session-rename-input {
   text-transform: uppercase;
 }
 
+.badge-readonly {
+  background: var(--bg-tertiary);
+  color: var(--text-secondary);
+}
+
 /* ── Chat Pane ───────────────────────────────────────────────── */
 .chat-pane {
   display: flex;
@@ -432,7 +701,8 @@ html[data-theme="light"] .session-rename-input {
 /* ── Chat Header Strip ───────────────────────────────────────── */
 .chat-header {
   display: flex;
-  justify-content: space-between;
+  justify-content: flex-start;
+  gap: 8px;
   align-items: center;
   padding: 7px 12px;
   background: var(--bg-secondary);
@@ -982,6 +1252,35 @@ html[data-theme="light"] .settings-textarea:focus {
   border-color: var(--accent);
 }
 
+/* ── Phase model override rows ───────────────────────────────── */
+.settings-phase-model-list {
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+  margin-top: 8px;
+}
+
+.settings-phase-model-row {
+  display: grid;
+  grid-template-columns: 80px 1fr auto;
+  align-items: center;
+  gap: 8px;
+}
+
+.settings-phase-model-label {
+  font-size: 12px;
+  font-weight: 600;
+  color: var(--text-secondary);
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+}
+
+.settings-phase-model-actions {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+}
+
 /* ── Settings Actions Row ────────────────────────────────────── */
 .settings-actions {
   display: flex;