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, 231 insertions, 0 deletions

diff --git a/renderer/src/components/Sidebar.tsx b/renderer/src/components/Sidebar.tsx
new file mode 100644
index 0000000..567e731
--- /dev/null
+++ b/renderer/src/components/Sidebar.tsx
@@ -0,0 +1,231 @@
+import React, { useState, useRef } from "react";
+import type { Project, Session } from "../types";
+import { formatRelativeTime } from "../utils/timeFormat";
+
+interface SidebarProps {
+  projects: Project[];
+  sessions: Session[];
+  selectedProject: Project | null;
+  selectedSession: Session | null;
+  onSelectProject: (project: Project) => void;
+  onSelectSession: (session: Session | null) => void;
+  onCreateProject: () => void;
+  onCreateSession: (projectId: string) => void;
+  onDeleteProject: (id: string) => void;
+  onDeleteSession: (id: string) => void;
+  onRenameSession: (id: string, name: string) => void;
+  loadingBySession: Record<string, boolean>;
+  width: number;
+  collapsed: boolean;
+  onCollapsedChange: (collapsed: boolean) => void;
+}
+
+export function Sidebar({
+  projects,
+  sessions,
+  selectedProject,
+  selectedSession,
+  onSelectProject,
+  onSelectSession,
+  onCreateProject,
+  onCreateSession,
+  onDeleteProject,
+  onDeleteSession,
+  onRenameSession,
+  loadingBySession,
+  width,
+  collapsed,
+  onCollapsedChange,
+}: SidebarProps) {
+  const [renamingSessionId, setRenamingSessionId] = useState<string | null>(null);
+  const [renameValue, setRenameValue] = useState("");
+  // Guard against double-commit (onKeyDown Enter → unmount → onBlur)
+  const renameCommitted = useRef(false);
+
+  const startRename = (session: Session) => {
+    renameCommitted.current = false;
+    setRenameValue(session.name);
+    setRenamingSessionId(session.id);
+  };
+
+  const commitRename = (sessionId: string) => {
+    if (renameCommitted.current) return;
+    renameCommitted.current = true;
+    const trimmed = renameValue.trim();
+    if (trimmed) onRenameSession(sessionId, trimmed);
+    setRenamingSessionId(null);
+  };
+
+  const cancelRename = () => {
+    renameCommitted.current = true;
+    setRenamingSessionId(null);
+  };
+
+  const handleDeleteProject = (project: Project) => {
+    if (confirm(`Delete project "${project.name}"? This cannot be undone.`)) {
+      onDeleteProject(project.id);
+    }
+  };
+
+  const handleDeleteSession = (session: Session) => {
+    if (confirm(`Delete session "${session.name}"? This cannot be undone.`)) {
+      onDeleteSession(session.id);
+    }
+  };
+
+  if (collapsed) {
+    return (
+      <div className="sidebar collapsed">
+        <button
+          className="sidebar-collapse-btn"
+          onClick={() => onCollapsedChange(false)}
+          title="Expand sidebar"
+        >
+          ▶
+        </button>
+      </div>
+    );
+  }
+
+  return (
+    <div className="sidebar" style={{ width }}>
+      {/* ── Header ── */}
+      <div className="sidebar-header">
+        <span className="sidebar-title">Projects</span>
+        <div className="sidebar-header-actions">
+          <button
+            className="sidebar-action-btn"
+            onClick={onCreateProject}
+            title="Add project"
+          >
+            +
+          </button>
+          <button
+            className="sidebar-collapse-btn"
+            onClick={() => onCollapsedChange(true)}
+            title="Collapse sidebar"
+          >
+            ◁
+          </button>
+        </div>
+      </div>
+
+      {/* ── Tree ── */}
+      <div className="sidebar-tree">
+        {projects.length === 0 && (
+          <div className="sidebar-empty">No projects yet</div>
+        )}
+
+        {projects.map((project) => {
+          const projectSessions = sessions.filter(
+            (s) => s.project_id === project.id
+          );
+          const isSelectedProject = selectedProject?.id === project.id;
+
+          return (
+            <React.Fragment key={project.id}>
+              {/* Project row */}
+              <div
+                className={`project-item${isSelectedProject ? " selected" : ""}`}
+              >
+                <span
+                  className="project-name"
+                  onClick={() => {
+                    onSelectProject(project);
+                    onSelectSession(null);
+                  }}
+                  title={project.path}
+                >
+                  {project.name}
+                </span>
+                <div className="item-controls">
+                  <button
+                    className="item-btn"
+                    onClick={() => onCreateSession(project.id)}
+                    title="New session"
+                  >
+                    +
+                  </button>
+                  <button
+                    className="item-btn item-btn-danger"
+                    onClick={() => handleDeleteProject(project)}
+                    title="Delete project"
+                  >
+                    ×
+                  </button>
+                </div>
+              </div>
+
+              {/* Empty sessions hint (only under selected project) */}
+              {projectSessions.length === 0 && isSelectedProject && (
+                <div className="sidebar-empty session-empty">
+                  No sessions yet
+                </div>
+              )}
+
+              {/* Session rows */}
+              {projectSessions.map((session) => {
+                const isSelected = selectedSession?.id === session.id;
+                const isLoading = loadingBySession[session.id] ?? false;
+                const isRenaming = renamingSessionId === session.id;
+
+                return (
+                  <div
+                    key={session.id}
+                    className={`session-item${isSelected ? " selected" : ""}`}
+                  >
+                    {isRenaming ? (
+                      <input
+                        className="session-rename-input"
+                        autoFocus
+                        value={renameValue}
+                        onChange={(e) => setRenameValue(e.target.value)}
+                        onKeyDown={(e) => {
+                          if (e.key === "Enter") commitRename(session.id);
+                          if (e.key === "Escape") cancelRename();
+                        }}
+                        onBlur={() => commitRename(session.id)}
+                      />
+                    ) : (
+                      <>
+                        <span
+                          className="session-name"
+                          onClick={() => onSelectSession(session)}
+                          title={`${session.name} · ${session.phase} · ${formatRelativeTime(session.updated_at)}`}
+                        >
+                          {session.name}
+                          {isLoading && (
+                            <span
+                              className="session-activity-dot"
+                              title="Thinking…"
+                            />
+                          )}
+                        </span>
+                        <div className="item-controls">
+                          <button
+                            className="item-btn"
+                            onClick={() => startRename(session)}
+                            title="Rename session"
+                          >
+                            ✏️
+                          </button>
+                          <button
+                            className="item-btn item-btn-danger"
+                            onClick={() => handleDeleteSession(session)}
+                            title="Delete session"
+                          >
+                            ×
+                          </button>
+                        </div>
+                      </>
+                    )}
+                  </div>
+                );
+              })}
+            </React.Fragment>
+          );
+        })}
+      </div>
+    </div>
+  );
+}