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]>

packages/contracts/src/api.ts

@@ -1540,10 +1540,20 @@ export const Api = {
       path: "/files/download",
       scope: "admin",
       auth: "session",
-      summary: "下载小文件（nfo/字幕/图片，≤20 MiB；流式；仅管理员控制台会话；审计）",
+      summary: "下载小文件（nfo/字幕/图片/文本/.torrent，≤20 MiB；流式；仅管理员控制台会话；审计）",
       query: files.FilesDownloadQuery,
       response: files.FilesDownloadResponse,
     }),
+    preview: ep({
+      method: "get",
+      path: "/files/preview",
+      scope: "admin",
+      auth: "session",
+      summary:
+        "结构化预览（文本/NFO 前 1 MiB 带编码探测、字幕前 200 条、.torrent 文件列表、图片尺寸；仅管理员控制台会话）",
+      query: files.FilesPreviewQuery,
+      response: files.FilePreview,
+    }),
     records: ep({
       method: "get",
       path: "/files/records",