feat(ui): sidebar project tree, status bar and a tabbed workbench for workspaces

The Workspace page's lane strip moves into the sidebar as a project tree:
workspaces grouped by git repository (new GET /api/projects, cached in
memory, POST /api/projects/refresh), untracked worktrees with Adopt, inline
rename, drag/Alt+arrow ordering and a right-click row menu. The selected
workspace is the URL's ?lane=, one shared lane list feeds the tree, the new
bottom status bar and the page. The Changes panel gains an activity bar
(Files / Changes / Sessions, with resume), and split panes get a header.

client/src/lib/projectTreePrefs.ts

@@ -0,0 +1,68 @@
+/**
+ * @file projectTreePrefs — the sidebar project tree's viewer preferences, in
+ * ONE localStorage key (`agenthub.sidebar.projects`): which projects are
+ * collapsed, whether a repo's untracked worktrees are listed, and which
+ * worktree sources (`claude` = `<repo>/.claude/worktrees/*`, `agenthub` =
+ * LANES_ROOT) are hidden. A `useSyncExternalStore` reader that caches nothing
+ * but the parsed blob for the current raw string, so the sidebar and Settings
+ * can never disagree. Nothing here reaches the server.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+import { useSyncExternalStore } from "react";
+
+export const PROJECT_TREE_KEY = "agenthub.sidebar.projects";
+
+export interface ProjectTreePrefs {
+  collapsed: Record<string, boolean>;
+  showUntracked: boolean;
+  hideSources: string[];
+}
+
+const DEFAULTS: ProjectTreePrefs = { collapsed: {}, showUntracked: true, hideSources: [] };
+const listeners = new Set<() => void>();
+let cache: { raw: string | null; value: ProjectTreePrefs } = { raw: null, value: DEFAULTS };
+
+function readRaw(): string | null {
+  try {
+    return localStorage.getItem(PROJECT_TREE_KEY);
+  } catch {
+    return null;
+  }
+}
+
+export function readProjectTreePrefs(): ProjectTreePrefs {
+  const raw = readRaw();
+  if (raw === cache.raw) return cache.value;
+  let value = DEFAULTS;
+  try {
+    const parsed = raw ? (JSON.parse(raw) as Partial<ProjectTreePrefs>) : {};
+    value = {
+      collapsed: parsed.collapsed && typeof parsed.collapsed === "object" ? parsed.collapsed : {},
+      showUntracked: parsed.showUntracked !== false,
+      hideSources: Array.isArray(parsed.hideSources) ? parsed.hideSources : [],
+    };
+  } catch {
+    value = DEFAULTS;
+  }
+  cache = { raw, value };
+  return value;
+}
+
+export function writeProjectTreePrefs(patch: Partial<ProjectTreePrefs>): void {
+  try {
+    localStorage.setItem(PROJECT_TREE_KEY, JSON.stringify({ ...readProjectTreePrefs(), ...patch }));
+  } catch {
+    /* disabled storage: the tree just forgets between visits */
+  }
+  listeners.forEach((l) => l());
+}
+
+function subscribe(l: () => void) {
+  listeners.add(l);
+  return () => listeners.delete(l);
+}
+
+export function useProjectTreePrefs(): ProjectTreePrefs {
+  return useSyncExternalStore(subscribe, readProjectTreePrefs);
+}