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-tauri/Cargo.toml

@@ -23,11 +23,15 @@ debug = "line-tables-only"
 tauri-build = { version = "2", features = [] }
 
 [dependencies]
-# protocol-asset serves downloaded media and cached thumbnails to the webview
-# over http://asset.localhost; without it convertFileSrc yields a URL nothing
-# answers. Paired with app.security.assetProtocol in tauri.conf.json, which
-# scopes it to $APPDATA/**.
-# TRACES: UR-071 | DR-134
+# protocol-asset serves cached thumbnails to the webview (asset://localhost on
+# Linux/macOS, http://asset.localhost on Windows/Android); without it
+# convertFileSrc yields a URL nothing answers. Paired with
+# app.security.assetProtocol in tauri.conf.json, which scopes it to
+# $APPDATA/thumbnails/** — the one directory still read through this protocol.
+# Downloaded media went the same way until DR-137 moved it to the loopback media
+# server, so the database, the encrypted-token fallback file and downloads/ are
+# all outside the grant now.
+# TRACES: UR-012, UR-071 | DR-134, DR-137, DR-198
 tauri = { version = "2", features = ["protocol-asset"] }
 tauri-plugin-opener = "2"
 tauri-plugin-os = "2"