Fix torrent loss on restart, session-shutdown ANR, and player icon overlap

[jpcottin]

Problem

1. Torrents disappeared after app restart even though their data was still on disk.
2. ANR when interacting with the app right after the session was recreated (e.g. after granting All-Files-Access).
3. Player CC button rendered as an unreadable blob — the subtitle and audio-track buttons were stacked on top of each other.

Root causes & fixes

Resume-data persistence

• onStop saved resume data on lifecycleScope (cancelled at ON_DESTROY) while onDestroy released the session on the main thread; if release won the race, nothing was saved. A new SessionLifecycleCoordinator serializes init/save/release on a FIFO queue owned by a process-wide scope — release can never run before a pending save.
• Resume files were written non-atomically; a process kill mid-write truncated previously-good files. Writes now use tmp + rename; corrupt files are quarantined as .corrupt and recovered via magnet re-add (the filename embeds the info-hash, so libtorrent rechecks the data already on disk).
• Resume data was only saved at onStop; it is now also requested eagerly on add_torrent, metadata_received, and torrent_finished alerts.

ANR

• nativeRelease held the global native mutex while libtorrent waited on tracker stop-announces (~27 s observed), blocking a main-thread addMagnet call. It now abort()s under the lock and finishes shutdown on a detached thread via session_proxy (re-init latency went from ~27 s to ~1 ms).
• All native calls moved off the main thread: DefaultDataRepository now owns the threading contract, routing every mutating call through the session queue — which also waits for pending init, so calls can no longer hit an uninitialized session.

UI

• Player: subtitle/audio buttons Box → Row (they were overlapping).
• Copy magnet: no longer silently writes an empty string to the clipboard; shows a toast on failure.
• Removed dead background-mode plumbing (onAppBackground/onAppForeground, setBackgroundMode).

Tests

• 9 new JVM unit tests for the coordinator, including a regression test for the save/release race (onDestroy_neverReleasesSessionBeforePendingSave).
• 5 new instrumented persistence tests: restart round-trip, eager resume write, corrupt-file quarantine, magnet recovery, removed-torrent non-resurrection.
• Full suite green: 29 JVM + 38 instrumented on an Android 15 emulator. CC fix verified visually on-device.