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/keymap.ts

@@ -37,6 +37,7 @@ export type KeyScope =
   | "ytPlayer"
   | "plexPlayer"
   | "mediaPlayer"
+  | "fileViewer"
   | "plexBrowse";
 
 const PAGE_LEVEL: ReadonlySet<KeyScope> = new Set<KeyScope>([
@@ -234,6 +235,10 @@ export const SHORTCUTS = [
   },
   { id: "mute", scope: "mediaPlayer", keys: ["m"], labelKey: "keys.mute" },
 
+  // --- the Files viewer: through the list's files that are not media ---------------------------
+  { id: "prevFile", scope: "fileViewer", keys: ["ArrowLeft"], labelKey: "keys.file" },
+  { id: "nextFile", scope: "fileViewer", keys: ["ArrowRight"], labelKey: "keys.file" },
+
   // --- the Plex browser (drill-down) ----------------------------------------------------------
   { id: "back", scope: "plexBrowse", keys: ["Backspace"], labelKey: "keys.browseBack" },
 ] as const satisfies readonly Shortcut[];