feat(files): load the Files panel one folder at a time, sort it, filter locally

[shenlvkang-collab]

Stacked on #631 (path picker download/upload). Review that one first; this PR's own change is the last commit, e52338d.

Problem

Opening the Files panel walks the whole workspace (depth 5, up to 5,000 nodes) on every open, refresh and session switch. Every keystroke in its search box (after the 250 ms debounce) walks it again on the server. On a slow mount this is the whole cost of the panel. Measured on WSL /mnt/d (9P, a stat per entry) against one real workspace (3,723 files, 1,277 dirs, truncated):

	Files panel today	Path picker, same root
open	13.6 s / 14.1 s, 645 KB	0.34 s / 0.47 s, 15 KB
one search	4.3 s	—

What changed

• Lazy listing: GET /api/sessions/:id/files?lazy=1[&path=<workspace-relative dir>] lists one directory.
  • Folders come back without children, which means "not loaded".
  • Every entry carries mtimeMs.
  • path is workspace-confined through validateSessionFilePath() (realpath).
  • A hidden segment needs showHidden.
  • The generated-folder excludes do not apply, since nothing is walked: node_modules costs one row.
  • The non-lazy response is byte-for-byte unchanged.
• Panel: loads the root lazily and fetches a folder when it is expanded.
  • A loading row shows while it loads; a failure row says why and retries on collapse and expand.
  • The same treeEpoch/owner/showHidden guards as the root load, so a listing that lands after a refresh, session switch or hidden toggle is dropped.
• Sort menu: name A→Z / Z→A, newest / oldest first, folders always first. Per device, the same modes as the path picker.
• Search: typing filters the nodes already loaded, in the browser, with no request (ordered-substring match with *, never a RegExp).
  • Status reads N matches in loaded folders · Enter searches the whole workspace.
  • Enter runs the existing server search (COD-236/341), unchanged.
  • A folder hit that the lazy tree has not loaded yet opens by loading each ancestor in turn.
• Expand All opens only loaded folders, so it never fans out into one request per folder.

Validation

• New test/routes/file-tree-lazy.test.ts (6, real temp dirs) covers: one-level shape and order, mtime, generated folders listed, subfolder paths, hidden opt-in, traversal and symlink escape refused, missing folder (404) and a file (400), and the unchanged default tree.

• New lazy block in test/file-browser-search.test.ts (6) covers:

  • expand-once loading
  • a stale listing after refresh is dropped
  • error then retry
  • date sort with folders first, persisted
  • local filter with no request, unloaded folders not searched, and Enter → server search
  • lazy reveal of an unloaded search hit

  The existing 94 tests in that file pass; 9 expected root-load URLs changed from depth=5 to lazy=1.

• The file-routes, path-picker, hidden-toggle and five i18n suites pass (292 tests across 14 files). Typecheck, lint, Prettier and frontend syntax pass.

• Playwright against a built server, 13 checks:

  • the root load is lazy and only the top level shows
  • oldest-first sort works
  • expanding fetches one folder
  • the local filter sends no request and shows loaded matches with their ancestors
  • a file in an unloaded folder is not found locally
  • Enter's workspace search finds it
  • clearing restores the tree
  • no page errors

🤖 Generated with Claude Code