Honor user video shortcuts in fullscreen video window

[niksedk]

Summary

• Fixes [SE5 RC2] [BUG] Video jump time is wrong in full screen for libmpv player #11392: in fullscreen, Right/Left arrows always jumped 2 sec and Up/Down always changed volume by 2, regardless of the user's configured "1 sec / 100 ms / 5 sec" jump bindings.
• Root cause: FullScreenVideoWindow's KeyDown handler hardcoded these keys before its local shortcut manager, and the local shortcut manager only knew about two commands (toggle fullscreen, show media info). Reordering alone wasn't enough — the user-configured shortcuts had no path to fire.

Changes

• MainViewModel.VideoFullScreen now builds an extraBindings list of the relevant video commands and passes it to FullScreenVideoWindow. The commands themselves operate on GetVideoPlayerControl(), which already returns _fullScreenVideoPlayerControl while fullscreen is open, so no per-fullscreen variants are needed.
  • Commands forwarded: Video100Ms{Back,Forward}, Video500Ms{Back,Forward}, VideoOneSecond{Back,Forward}, VideoOneFrame{Back,Forward}, VideoMoveCustom1{Back,Forward}, VideoMoveCustom2{Back,Forward}, Play, Pause, TogglePlayPause, TogglePlayPause2, VideoToggleBrightness.
• FullScreenVideoWindow gains an optional extraBindings constructor parameter, registers each into the local shortcut manager, and the KeyDown handler is reordered:
  1. Esc/F11 — always close.
  2. User-configured shortcuts — first chance to claim the key.
  3. Hardcoded Space / Right / Left / Up / Down — fallbacks only when no user binding matched.
• Volume Up/Down stay hardcoded because there are no Volume RelayCommands in the view model; the fullscreen defaults are the only keyboard-volume path.

Test plan

• In Options > Shortcuts, set Right = "Video 1 sec forward" and Left = "Video 1 sec back". Enter fullscreen and confirm each press moves exactly 1 sec.
• Repeat with "100 ms" and "500 ms" bindings — confirm the jumps match the configured magnitude.
• With no Right/Left binding (or bindings cleared), confirm fullscreen still falls back to the original 2-second jumps.
• Confirm Space play/pause still works (with and without a TogglePlayPause binding).
• Confirm Up/Down still change volume by 2.
• Confirm Esc and F11 still exit fullscreen.
• Confirm the existing fullscreen-toggle key (e.g. F) still exits fullscreen (and the focus restoration from [SE5 RC2] [BUG] Video full screen shortcut works only once #11393 still works).
• Confirm Show Media Information shortcut still opens with the fullscreen window as owner.

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