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/components/lanes/LaneChangesPanel.tsx

@@ -8,7 +8,9 @@
  *   file     — one file, as a diff or as its whole content (issue #35); a
  *              whole markdown file can also be read rendered (Preview)
  *   history  — recent commits; opening one shows that commit's diff
- *   browse   — quick-open by path, or the repo's tracked file tree
+ *   browse   — quick-open by path, or the repo's tracked file tree (the
+ *              Workspace right panel's Files tab; the other four are its
+ *              Changes tab)
  *
  * and three ranges: `work` (uncommitted, what the agent has touched right
  * now), `branch` (`base...HEAD`, what this lane adds on top of its base) and
@@ -417,8 +419,13 @@ export function LaneChangesPanel({
   laneId,
   onNewLane,
   full = false,
+  tab = "changes",
 }: {
   laneId: number;
+  /** The Workspace right panel's activity-bar tab: `files` is the browse mode
+   *  on its own (no git header, no mode switcher), `changes` the other four.
+   *  At full width both show the explorer sidebar beside the changes. */
+  tab?: "files" | "changes";
   /** Open the Add-lane flow for a sibling worktree; see `LaneGitBar`. */
   onNewLane?: (branch?: string) => void;
   /** The panel has the whole width: the file tree becomes a sidebar and a
@@ -582,16 +589,19 @@ export function LaneChangesPanel({
       .catch((err) => setError(err instanceof Error ? err.message : String(err)));
   }, [mode, laneId, reread]);
 
+  // Re-run when a lane switch empties the cache too: the Files tab stays in
+  // `browse` across the switch, so the mode alone never changes.
+  const treeRootLoaded = "" in treeChildren;
   useEffect(() => {
     const wanted = full || (mode === "browse" && !query.trim());
-    if (!wanted || "" in treeChildren) return;
+    if (!wanted || treeRootLoaded) return;
     void api.lanes
       .gitTree(laneId, "")
       .then(forLane(laneId, (r) => setTreeChildren((prev) => ({ ...prev, "": r.entries }))))
       .catch((err) => setError(err instanceof Error ? err.message : String(err)));
-    // `treeChildren` is the cache this fills — reading it as a dep would loop.
+    // `treeChildren` itself is the cache this fills — a dep on it would loop.
     // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, [mode, laneId, query, reread, full]);
+  }, [mode, laneId, query, reread, full, treeRootLoaded]);
 
   // A folder's children are fetched lazily, the first time it is expanded.
   const toggleDir = (path: string) => {
@@ -706,6 +716,11 @@ export function LaneChangesPanel({
     returnTo.current = null;
     pendingScroll.current = null;
   }, [laneId]);
+  // After the lane reset above: each tab opens on its own mode.
+  useEffect(() => {
+    if (!full) setMode(tab === "files" ? "browse" : "overview");
+  }, [tab, laneId, full]);
+  const filesTab = tab === "files" && !full;
 
   // Restoring has to wait for the list to be TALL enough again: leaving a file
   // re-fetches the mode it came from, so the first render back is an empty
@@ -1032,7 +1047,9 @@ export function LaneChangesPanel({
         </aside>
       )}
       <div className="flex min-h-0 min-w-0 flex-1 flex-col">
-        <LaneGitHeader laneId={laneId} git={git} onChanged={rereadAll} onNewLane={onNewLane} />
+        {!filesTab && (
+          <LaneGitHeader laneId={laneId} git={git} onChanged={rereadAll} onNewLane={onNewLane} />
+        )}
         {/* ONE wrapping row: where you are (mode) then how you are looking at it
           (range, layout, context, whole file), refresh last. Each group is
           its own inline box, so a narrow panel wraps BETWEEN groups — never
@@ -1048,12 +1065,9 @@ export function LaneChangesPanel({
                 <ChevronLeft className="h-3 w-3" />
                 {t("changes.back")}
               </button>
-            ) : (
+            ) : filesTab ? null : (
               <div className="flex min-w-0 items-center rounded border border-border bg-surface-2 p-0.5">
-                {(full
-                  ? (["overview", "review", "history"] as const)
-                  : (["browse", "overview", "review", "history"] as const)
-                ).map((m) => (
+                {(["overview", "review", "history"] as const).map((m) => (
                   <button
                     key={m}
                     type="button"