feat(lanes): full-width Changes, Stage all, one-read expand-all

- The Changes panel's width control gains a full-width step (narrow →
  50/50 → 70/30 → full → closed); the console beside it is hidden with
  CSS, never unmounted.
- "Expand all folders" is one GET /api/lanes/:id/git/tree-all (one
  `git ls-files`, every level shaped like /git/tree) instead of one
  request and one git process per folder.
- Overview: a Stage all button and the staged-only commit box sit above
  the file list and stay pinned while it scrolls; the Files finder is
  pinned too.

client/src/i18n/locales/en/lanes.json

@@ -110,6 +110,8 @@
   "changes.size.compact": "Narrow",
   "changes.size.half": "50 / 50",
   "changes.size.wide": "70 / 30",
+  "changes.size.full": "Full width",
+  "changes.stageAll": "Stage all ({{count}})",
   "changes.stage": "Stage this file",
   "changes.title": "Changes",
   "changes.truncated": "Some changes were left out to keep this panel responsive.",

client/src/i18n/locales/vi/lanes.json

@@ -110,6 +110,8 @@
   "changes.size.compact": "Hẹp",
   "changes.size.half": "50 / 50",
   "changes.size.wide": "70 / 30",
+  "changes.size.full": "Toàn màn hình",
+  "changes.stageAll": "Stage tất cả ({{count}})",
   "changes.stage": "Đưa file này vào index",
   "changes.title": "Thay đổi",
   "changes.truncated": "Một phần thay đổi đã bị lược bớt để bảng này còn mượt.",