feat(files): a file that is not media opens in a viewer — an image, a PDF, text as text

- GET /api/files/{id}/preview: an allow-list on the stored extension (downloads/preview.py) picks
  image / PDF / text; served inline under `CSP: sandbox; frame-ancestors 'self'`, the type from the
  table; HTML and SVG arrive as plain text; a text is its first 1 MiB (X-Preview-Truncated), sent
  as UTF-8 whatever it was written in (BOM, UTF-8, else cp1250); a NUL means binary: 415
- the list says which preview each file gets (asset.preview); FE/BE contract on kinds and header
- the viewer (shared dialog shell): ← / → through the list's such files in its order, Save, open in
  a tab, the file's menu; one stage height for every kind; a no-preview card offers the file
- the name, the card's picture and the menu's Preview open it; an image up to 8 MB is its own
  thumbnail

frontend/src/lib/api/downloads.ts

@@ -39,9 +39,14 @@ interface DownloadAsset {
   // (video/audio) gets play/edit; a generic uploaded file is download+share only, no player. Null
   // while still in flight (set at finalize). See backend downloads/mediakind.py.
   media_kind: "video" | "audio" | "file" | null;
+  // What the library's viewer shows of a generic file (backend downloads/preview.py): an image, a PDF
+  // in the browser's own viewer, plain text; null = no preview (media plays instead).
+  preview?: FilePreviewKind | null;
   thumbnail_url: string | null;
 }
 
+export type FilePreviewKind = "image" | "pdf" | "text";
+
 export type DownloadStatus = "queued" | "running" | "paused" | "done" | "error" | "canceled";
 // "download" (default), "edit" (a per-user trim/crop derivative cut from another job), "upload"
 // (a locally-uploaded file) or "clip" (a marked range of a YouTube video or a Plex item, cut straight