Add Episodes panel: search a series, browse a season's episode list

New src-tauri/src/episodes.rs exposes browse_episodes, reusing the
existing AnyTvClient::fetch_season machinery Fetch Data already built
(via a build_tv_client helper factored out of metadata.rs) so this
panel needs no credentials of its own - it reads whatever TV source
(TMDB/TVmaze/OMDb) is already configured in Fetch Data Settings.

New EpisodesPanel.tsx/.css: series name + season stepper, an episode
list (number/title/air date), and a Copy list button that copies the
season as plain text. Wired into App.tsx's sidebar routing as the
second stub panel (after Subtitles) to become a real screen.

145/145 Rust tests passing (+1), npm run build clean, 15/15 Playwright
checks against npm run dev (mocked Tauri bridge) covering search,
the empty-season case, a not-found series, the season stepper, Copy
list, and the blank-query guard.

src-tauri/src/lib.rs

@@ -1,4 +1,5 @@
 mod cli;
+mod episodes;
 mod files;
 mod history;
 mod metadata;
@@ -115,6 +116,7 @@ pub fn run() {
             settings::save_opensubtitles_login,
             subtitles::search_subtitles,
             subtitles::download_subtitle,
+            episodes::browse_episodes,
             history::list_rename_history,
             history::undo_rename_batch,
             presets::list_presets,