feat(security): set a restrictive CSP and scope the asset protocol to thumbnails

`app.security.csp` was `null`, so the webview ran with no Content-Security-Policy
at all: any script that reached the web layer would have inherited the whole IPC
surface. There is no known injection path today (one app-owned `{@html}`, no
`innerHTML`/`eval`), so this is defence in depth rather than a fix for an open
hole.

`script-src 'self'` is the restrictive half — Tauri nonces SvelteKit's inline
bootstrap script at build time, so no `'unsafe-inline'` is needed — together with
`object-src`/`frame-src 'none'` and `base-uri 'self'`. `img-src`/`media-src`/
`connect-src` cannot be restrictive: the Jellyfin origin is typed in by the user
at run time and is routinely plain http on a LAN, so they allow `http:`/`https:`.
That is a wide grant for data, but it still bars `file:`/`filesystem:` and does
not touch script execution. A run-time policy naming the server exactly was
rejected: Tauri derives the header from immutable config when it serves the HTML,
so it would mean rebuilding config and reloading the webview on every server
change. `style-src` keeps `'unsafe-inline'` because Svelte compiles `style="…"`
attributes into markup; `worker-src`/`media-src` keep `blob:` for hls.js's
demuxer worker and its MSE object URL; `ipc:`/`http://ipc.localhost` keeps
`invoke` working. `devCsp` mirrors it with the eval/inline/websocket allowances
Vite's dev server needs.

The asset-protocol scope narrows from `$APPDATA/**` — the storage root holding
the SQLite database and the encrypted-token fallback file — to
`$APPDATA/thumbnails/**`. Since DR-137 moved downloaded media to the loopback
media server, `imageCache` is the only `convertFileSrc` caller left.

Needs manual verification on both platforms: thumbnails, online HLS video and
offline downloaded video cannot be exercised headlessly.

src/lib/services/imageCache.ts

@@ -41,7 +41,10 @@ export async function getCachedImageUrl(
     const cachedPath = await commands.thumbnailGetCached(itemId, imageType, tag);
 
     if (cachedPath) {
-      // Convert file path to asset URL for Tauri
+      // Convert file path to asset URL for Tauri. This is the only remaining
+      // convertFileSrc caller, which is why the asset-protocol scope is narrowed
+      // to $APPDATA/thumbnails/** — a path outside it resolves to nothing.
+      // TRACES: UR-012 | DR-134, DR-198
       return convertFileSrc(cachedPath);
     }
   } catch (e) {