Background update check, music extension detection, and misc fixes

[AeEn123AI]

Summary

• Background update check — the update check is moved onto a background thread so the main window opens immediately instead of blocking on startup. An in-app UpdatePrompt widget surfaces the result when an update is found.
• Music file extension detection — raw audio files (e.g. Roblox /sounds music stored without HTTP headers) now have their extension detected from magic bytes (OggS → .ogg, ID3/frame-sync → .mp3), falling back to .ogg for the Music tab.
• Asset lists backed by Arc (perf) — FILE_LIST / FILTERED_FILE_LIST are now Mutex<Arc<Vec<AssetInfo>>>. The GUI takes a cheap Arc snapshot each frame (a refcount bump) instead of deep-cloning the whole Vec and all its Strings on every repaint. Writes use Arc::make_mut (copy-on-write): zero clones in the steady state, and at most one clone per frame during a refresh versus the old unconditional full clone every frame. Kept dependency-free (no arc-swap crate) since the lock is uncontended and the binary is size-constrained.
• extract_bytes underflow fix — replaced direct subtraction with saturating_sub so a header found before the offset bytes cannot underflow the index and cause a slice panic.
• Tab keyboard navigation panic fix — egui::Key::from_name(...).expect(...) replaced with a from_name + continue guard so tabs beyond key 9 don't panic.
• filter_file_list simplification — filter collects into a Vec once, then assigns under a single lock, removing repeated per-entry lock acquisitions.
• Locale additions — new strings added across all supported locales (de-DE, en-GB, englifsh, ja-JP, pirate-speak, pl-PL, ru-RU, shakespearian-english, zh-CN).

Test plan

• Launch the app — window should appear immediately without waiting for the update check
• If an update is available, verify the in-app prompt appears
• Extract music assets and confirm .ogg / .mp3 extensions are set correctly
• Tab keyboard shortcuts (Ctrl/Alt + 1–9) work without panic
• Search/filter in the file list returns correct results

Verification performed

• cargo test — 22 passed (includes new test_file_list_arc_snapshot_and_filter covering the copy-on-write snapshot-stability guarantee the render loop relies on)
• End-to-end CLI run against an isolated cache: --list --mode music lists all assets; --extract --mode music --extension produces byte-for-byte-identical files with correct .ogg/.mp3 extensions detected from magic bytes

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