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/vitest.config.ts

@@ -8,6 +8,7 @@ import { defineConfig } from "vitest/config";
 // 新测试文件缺省归入 pg(串行,安全);确认不碰 PG 的才加进 UNIT。
 const UNIT = [
   "test/api-registry.test.ts",
+  "test/file-manager/preview-parse.test.ts",
   "test/auth.test.ts",
   "test/health.test.ts",
   "test/sources-identity/identity-differential.test.ts",