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.

docs/API.md

@@ -498,6 +498,7 @@ GET /api/lanes/:id/git/log?limit=30[&file=<path>]
 GET /api/lanes/:id/git/file?path=<path>[&rev=work|HEAD|<sha>]
 GET /api/lanes/:id/git/ls?q=<substring>
 GET /api/lanes/:id/git/tree?path=<dir>
+GET /api/lanes/:id/git/tree-all
 ```
 
 The reads behind the Changes panel's **History**, whole-file view, quick-open
@@ -532,6 +533,7 @@ and file explorer. All four are read-only and unguarded, and all four answer
 - **`/git/tree`** → `{available, path, entries:[{name, path, dir}], truncated}`
   — one directory level, tracked entries only, directories folded and listed
   first. `400 EBADPATH` for a path that escapes the lane.
+- **`/git/tree-all`** → `{available, levels: {"<dir>": [{name, path, dir}]}, truncated}` — every level of the tracked tree from ONE `git ls-files`, each level shaped and capped like `/git/tree`'s; the Files tab's **Expand all folders** is this one request instead of one per folder.
 
 #### Read a lane's own branches