feat: edit in the Changes panel, git graph history, plan quota (v2.3.0)

- Changes panel: a working file opens in a CodeMirror editor with its diff
  inline (DiffEditor.tsx, @codemirror/merge, lazy-loaded); Changes folds the
  unchanged lines, Whole file shows them; side by side at 50/50 and full
  width; a Revert per block; original from HEAD or the branch's merge base
  (`mergeBase` on GET /git/changes?range=branch)
- Every diff: green = line added whole, red = removed whole, blue = edited
  in place (--diff-mod)
- History drawn as a commit graph (#58: `parents` on GET /git/log,
  --date-order; lib/commitGraph.ts)
- Per-workspace never-stage list, drawn orange in the Files tree (--kept);
  closed Terminal rows past three deleted
- Status bar plan quota for Claude (per account) and Codex
  (GET /api/pricing/quota, server/lib/quota.js); compact hover panel that
  stays open, Refresh button with a spinner
- Settings → Appearance holds language, theme and pet; Ctrl+Shift+C copies;
  compact sidebar navigation and short session times

client/src/index.css

@@ -105,6 +105,10 @@
   --syn-plain: var(--fg-primary);
   --syn-add: 4 120 87; /* emerald-700 */
   --syn-del: 185 28 28; /* red-700 #b91c1c */
+  /* A diff line edited in place (not wholly added or removed). */
+  --diff-mod: 29 78 216; /* blue-700 #1d4ed8 */
+  /* A file on the Changes panel's never-stage list (orange, apart from amber M). */
+  --kept: 194 65 12; /* orange-700 #c2410c */
 }
 
 .dark {
@@ -154,6 +158,8 @@
   --syn-plain: 229 231 235; /* gray-200    #e5e7eb — kept off fg-primary so Default is unchanged */
   --syn-add: 110 231 183; /* emerald-300 */
   --syn-del: 252 165 165; /* red-300 #fca5a5 */
+  --diff-mod: 96 165 250; /* blue-400 #60a5fa */
+  --kept: 251 146 60; /* orange-400 #fb923c */
 }
 
 /* Named themes (`data-theme` on <html>, set by hooks/useTheme.ts, picked in