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/workflows/CompactionImpact.tsx

@@ -407,7 +407,7 @@ export function CompactionImpact({ data }: CompactionImpactProps) {
       {/* Hover tooltip (matches the app's other chart tooltips) */}
       {tip && (
         <div
-          className="fixed z-50 pointer-events-none rounded-md border border-[#2a2a4a] bg-[#12121f] px-2.5 py-1.5 text-xs shadow-xl"
+          className="fixed z-50 pointer-events-none rounded-md border border-border bg-surface-2 px-2.5 py-1.5 text-xs shadow-xl"
           style={{
             left: tip.x > window.innerWidth - 220 ? tip.x - 14 : tip.x + 14,
             top: tip.y - 10,