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/pages/Workflows.tsx

@@ -435,10 +435,10 @@ function ChartInfoPopover({ infoKey, title }: { infoKey: string; title: string }
         <div
           ref={popoverRef}
           role="tooltip"
-          className="fixed z-50 p-3.5 bg-[#12121f] border border-[#2a2a4a] rounded-lg shadow-2xl text-[11px] text-fg-secondary pointer-events-none"
+          className="fixed z-50 p-3.5 bg-surface-2 border border-border rounded-lg shadow-2xl text-[11px] text-fg-secondary pointer-events-none"
           style={{ left: coords.left, top: coords.top, width: POPOVER_W }}
         >
-          <p className="text-xs font-semibold text-fg-primary mb-2.5 pb-2 border-b border-[#2a2a4a]">
+          <p className="text-xs font-semibold text-fg-primary mb-2.5 pb-2 border-b border-border">
             {title}
           </p>