feat(files): wide file manager layout and basic previews (FM-2)

- Files page uses a wide layout (route handle layout:"wide"): no max width,
  full-height directory tree docked next to the sidebar, resizable
  (200-520px, remembered, double-click resets) and collapsible; mobile keeps
  the tree in a drawer.
- GET /files/preview (admin console session only, same path guards as
  download): text/NFO with encoding detection (BOM/UTF-16/UTF-8/GB18030/Big5),
  subtitles first 200 cues (SRT/VTT/ASS/SSA, PGS timings), .torrent file
  list/trackers (passkey masked)/infohash, image dimensions. Reads at most
  1 MiB of text; .torrent capped at 4 MiB.
- Download also allows text files, .torrent and avif/gif.
- Research notes in docs/design/file-preview-research.md.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/server/test/api-registry.test.ts

@@ -68,6 +68,7 @@ describe("Api 注册表鉴权标注", () => {
         "PATCH /auth/passkeys/{id}",
         "DELETE /auth/passkeys/{id}",
         "GET /files/download",
+        "GET /files/preview",
         "PATCH /approvals/{id}",
         "POST /approvals",
         "POST /auth/password",