feat: add YouTube-style keyboard shortcuts to video player

[polferov]

Summary

Adds keyboard shortcuts to the video player. Before this, a physical keyboard did nothing during playback; for example, Space did not pause on a tablet with a keyboard attached. The keymap copies the YouTube web player's shortcuts. The keys are handled in PlayerActivity.dispatchKeyEvent, so they work with both ExoPlayer and mpv. Every action goes through an existing PlayerEvent or ViewModel call, so sync-play, sleep-timer resume and the on-screen indicators behave exactly as they do with touch controls.

Changes

Player: keyboard shortcuts (PlayerActivity.kt)

Key	Action
Space / K	Play / pause and show the controls. Also pauses while buffering and resumes after the end. Space always toggles playback, even when a button has focus.
Enter	Presses the visible Skip Intro / Skip Outro button. When a control has focus, Enter clicks that control instead.
J / L	Seek −10 s / +10 s
← / →	Seek −5 s / +5 s
↑ / ↓	Volume up / down by one system step (shows the volume indicator)
M	Mute / unmute (goes back to the previous volume)
0–9 (top row, or numpad with NumLock on)	Jump to 0–90 %
Home / End	Jump to start / end
< / >	Speed −/+0.25 (0.25×–2×, same range as the speed panel); matched by typed character, so it works on QWERTZ and AltGr layouts
Shift + N / Shift + P	Next / previous episode (only when the queue has one)
I	Picture-in-picture

Which input gets shortcuts. Only physical full (alphabetic) keyboards. Remotes, gamepads, D-pads, HDMI-CEC remote input and virtual devices behave exactly as on master.

Keys that wait for typing. Arrows, digits and Home/End only become shortcuts after that same keyboard has pressed a letter or Space in the player. Until then they keep master's behaviour. Some TV remotes report as full keyboards and have arrow and number keys, so this keeps them working as before.

When shortcuts are off. Keys go to the framework untouched when:

• the player isn't ready, or no item is loaded,
• a cast session is connected,
• the controls lock is on,
• an in-window panel is open (tracks, quality, speed, episodes, chapters, sleep timer, SyncPlay members, version picker), or
• Ctrl, left Alt or Meta is held.

Live TV and intros. Seeking and episode skips are disabled there. Their keys are consumed but do nothing, so they don't turn into focus moves.

Focus. If a control has keyboard focus (after Tab), it gets each key first, except Space. Arrow keys stay with focus navigation even at the edge of the controls.

Key repeat. Holding a seek or volume key repeats it. Inside a SyncPlay group, held seek keys fire once per press, so they don't flood the server with seeks.

Panel-open signal (PlayerControls.kt, PlayerScreen.kt, PlayerViewModel.kt)

PlayerControls reports whether a panel is open through a new onPanelOpenChange callback. The check uses the same conditions each panel renders under. PlayerScreen adds the version picker and writes the result to PlayerViewModel.isOverlayPanelOpen, which the activity reads.

Test plan

• ./gradlew :app:compileDebugKotlin: passes
• ./gradlew :app:testDebugUnitTest: passes
• ./gradlew :app:assembleDebug: passes
• ./gradlew ktfmtCheck: none of the changed files is newly flagged. The task fails on 19 files, an identical list on master. PlayerViewModel.kt is on that list already, and its new lines are kept out of the formatter's rewrite.
• ./gradlew checkCancellationRethrow: fails on 2 sites in AudiobookshelfProgressSyncer.kt that fail identically on master. This diff doesn't touch them.
• Repeated independent code reviews, with fixes between rounds.
• Manual check on an Android tablet with a hardware keyboard (in progress). Feedback so far is already included: Space no longer presses Skip Outro, and pausing shows the controls.
• Manual test of every shortcut:
  • lock mode
  • intro / outro skip

Risk

• Contracts and deploy: no breaking API, schema or contract changes, no deploy ordering, no new configuration. The change is client-only.
• First arrow press: on a real keyboard, arrows, digits and Home/End do nothing until a letter or Space has been pressed in the player. Usually that's the first Space to pause.
• Keyboard focus while paused: after Tab, focus stays on the controls while paused, because the controls don't auto-hide then. Arrows move focus and Enter clicks the focused control, as in a browser. Touch or resuming playback leaves that state.
• Mute: M sets the system media volume to 0, the same mechanism the volume gesture uses. If you leave the player while muted, other media apps stay silent too.
• End key: End seeks to the end, as YouTube does, which can mark the item played, trigger autoplay or close the player on the last item.
• Already present, not changed here: Shift+N/P and the speed keys reuse onNextEpisode/onPreviousEpisode/SetPlaybackSpeed. Like the on-screen buttons and the speed panel, these act locally only inside a SyncPlay group.

[MakD]

@polferov is this ready for testing?

[polferov]

Yes. I've been using it for the last few days without problems.