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/Tip.tsx

@@ -4,6 +4,13 @@
  * by {@link StatCard} for raw metric values and anywhere a compact display needs
  * a full-precision expansion without cluttering the layout.
  *
+ * ## Look
+ * The bubble is {@link TOOLTIP_SURFACE} — the same surface the app-wide
+ * {@link GlobalTooltip} draws — so a raw-value tooltip and a control's tooltip
+ * are visibly one thing. It stays cursor-following (and not anchored like the
+ * global one) because it expands a NUMBER inside a line of text, where the
+ * anchor is a few characters wide and the cursor is the real subject.
+ *
  * ## Portal rendering
  * Tooltip content is portaled to `document.body` with `position: fixed` so
  * parent `overflow: hidden` cannot clip it. Placement flips left/up when the
@@ -56,6 +63,7 @@
 
 import { useState, useRef, useCallback } from "react";
 import { createPortal } from "react-dom";
+import { TOOLTIP_SURFACE } from "./Tooltip";
 
 /** Props for {@link Tip}. */
 interface TipProps {
@@ -135,11 +143,7 @@ export function Tip({ raw, children, maxWidth = 320, block = false }: TipProps)
       {children}
       {show &&
         createPortal(
-          <div
-            ref={tipRef}
-            style={tipStyle}
-            className="px-2.5 py-1.5 text-[11px] leading-relaxed font-mono text-fg-primary bg-[#12121f] border border-[#2a2a4a] rounded-lg shadow-xl pointer-events-none whitespace-pre-wrap break-words"
-          >
+          <div ref={tipRef} style={tipStyle} className={`${TOOLTIP_SURFACE} font-mono`}>
             {raw}
           </div>,
           document.body