feat(slint): offline cache manager + remove/redownload + Settings Offline page

Slices 3 + 4 of the offline-cache port.

Slice 3 (controller, offline_cache.rs):
- remove_album (maintenance::remove_album_cached_tracks + library rows + live
  row update), redownload_track / redownload_album(failed_only) (via
  select_redownload_targets + reset_track_for_redownload + spawn), clear_all
  (purge), open_folder (xdg-open / open the cache dir).

Slice 4 (manager + Settings):
- OfflineManagerView.slint: a standalone route (NavEntry::OfflineManager ->
  ContentView.offline-manager, mounted in AppShell, mirrors PlaylistManager).
  Stats bar (tracks count / size / limit / usage bar + edit-limit GB input +
  open-folder + clear-all) over a scrolling album-header + track-row list with
  a spinning/check/alert status glyph and per-row re-download / remove.
- offline_manager.rs: builds the album->track rollup + stats from index.db
  (get_all_tracks), pushes [OfflineRow] + stats to OfflineManagerState; set_limit
  (in-memory, session-scoped) + reload_soon (refresh after a mutation).
- OfflineManagerActions wired in main.rs (open/refresh/remove/redownload/
  set-limit/clear-all/open-folder/play).
- Settings gains an "Offline" section (OfflineSettings.slint): Manage offline
  cache (-> the manager), Open cache folder, Clear all. The Local-Library
  rows (show-in-library, repair) arrive with the Local Library port.

qbz-slint compiles clean.

cc

crates/qbz-slint/ui/shell/AppShell.slint

@@ -31,6 +31,7 @@ import { FavoritesView } from "../favorites/FavoritesView.slint";
 import { MixView } from "../mix/MixView.slint";
 import { PlaylistView } from "../playlist/PlaylistView.slint";
 import { PlaylistManagerView } from "../playlist/PlaylistManagerView.slint";
+import { OfflineManagerView } from "../offline/OfflineManagerView.slint";
 import { SettingsView } from "../settings/SettingsView.slint";
 import { SearchResultsView } from "../search/SearchResultsView.slint";
 
@@ -169,6 +170,7 @@ export component AppShell inherits Rectangle {
                     }
                 }
                 if NavState.view == ContentView.playlist-manager: PlaylistManagerView { }
+                if NavState.view == ContentView.offline-manager: OfflineManagerView { }
                 if NavState.view == ContentView.mix: MixView {
                     play-track(id) => {
                         root.media-action("mix-track", id, "play");