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/components/lanes/__tests__/LaneCard.test.tsx

@@ -216,6 +216,12 @@ describe("LaneCard git block", () => {
       { path: "client/src/lib/api.ts", status: "M ", added: 1, deleted: 0 },
       { path: "scratch.txt", status: "??", added: null, deleted: null },
     ],
+    // Tracking facts ride along on the same endpoint; a branch that has never
+    // been pushed reports nulls rather than zeros.
+    hasRemote: true,
+    upstream: null,
+    ahead: null,
+    behind: null,
   };
 
   it("renders the live branch, head, subject and uncommitted counts", async () => {