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.

server/routes/lanes.js

@@ -41,7 +41,14 @@ const {
   resolveBase,
   slugify,
 } = require("../lib/worktree");
-const { collectChanges, listCommits, listPaths, listTree, readFileAt } = require("../lib/git-diff");
+const {
+  collectChanges,
+  listCommits,
+  listPaths,
+  listTree,
+  listTreeAll,
+  readFileAt,
+} = require("../lib/git-diff");
 const { runGitAction } = require("../lib/git-ops");
 const { withLaneLock } = require("../lib/lane-lock");
 const { checkSync, mergeSync, continueSync } = require("../lib/lane-sync");
@@ -859,6 +866,17 @@ router.get("/:id/git/ls", async (req, res) => {
   }
 });
 
+/** Every level of the tracked tree in one read, for "expand all folders". */
+router.get("/:id/git/tree-all", async (req, res) => {
+  const lane = laneOr404(req, res);
+  if (!lane) return;
+  try {
+    res.json({ available: true, ...(await listTreeAll(lane.cwd)) });
+  } catch {
+    res.json({ available: false, levels: {}, truncated: false });
+  }
+});
+
 /** One directory level of tracked files, for the file explorer. */
 router.get("/:id/git/tree", async (req, res) => {
   const lane = laneOr404(req, res);