feat(lanes,ui): per-lane git surface — Changes panel, VS Code-style diff, branch-aware Add lane, redesigned tooltips (#34, #35)

- Changes card beside lane detail with five modes: Files tree, overview,
  Review (whole diff, per-file collapse), single file, commit history.
- LaneDiffView: unified / side-by-side with independent scroll on both axes
  and mirrored scrollLeft, expand/collapse context, intra-line emphasis.
- Staged-only commit box; git actions (fetch/pull/push/stage/unstage/
  discard/commit/switch/branch-create/branch-delete) guarded in git-ops.js.
- Server: git-diff.js + git-ops.js behind GET /api/lanes/:id/git/* and
  POST /api/lanes/:id/git/:action; gitFacts gains upstream/ahead/behind.
- Add lane: new/existing branch, current branch, worktree list, prefill
  from the git bar's "New lane here"; /api/run/cwds gains a repo kind.
- GlobalTooltip upgrades every native title into a themed anchored bubble;
  chart/info popovers use theme tokens.

client/src/App.tsx

@@ -60,6 +60,7 @@ import { useCallback } from "react";
 import { Layout } from "./components/Layout";
 import { DocumentTitle } from "./components/DocumentTitle";
 import { SplashScreen } from "./components/SplashScreen";
+import { GlobalTooltip } from "./components/Tooltip";
 import { Dashboard } from "./pages/Dashboard";
 import { History } from "./pages/History";
 import { SessionDetail } from "./pages/SessionDetail";
@@ -87,6 +88,9 @@ export default function App() {
   return (
     <>
       <SplashScreen />
+      {/* One renderer gives every `title` in the app the designed bubble —
+          see components/Tooltip.tsx for why it is not a wrapper component. */}
+      <GlobalTooltip />
       <BrowserRouter>
         <DocumentTitle />
         <Routes>