fix(playback): make playback keyboard shortcuts work without shared controls (#1398)

* fix(playback): make playback keyboard shortcuts work without shared controls

With the default configuration (Video.js, webPlayerSharedControls off) the
playback shortcuts advertised in the in-app help and README — Space/K, F,
arrow seek/volume, M — silently did nothing: ControlsShortcuts only exists
inside app-player-controls, which never renders on the preference-off path.

Attach a LegacyPlayerShortcuts wrapper (same arbitration and ignore rules)
in the vendor-chrome HTML5, Video.js, and ArtPlayer players, forwarding the
commands to each engine's own API. Seek stays gated on authoritative VOD
metadata plus a finite positive duration, and a visible playback diagnostic
disables the keys. The legacy ArtPlayer chrome now passes hotkey:false —
its focus-scoped vendor hotkeys ignore defaultPrevented and would
double-handle every key — with its Escape-exits-web-fullscreen behavior
restored by the new wiring.

The playback entries in the in-app shortcut help and README drop their
embedded-MPV-only qualifier, since the keys now work in every runtime.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014h2cZi5DcFSbcmV7WgB6qB

* fix(playback): restore audible volume when M unmutes at zero volume

Addresses the Codex review finding on #1398: after arrowing the volume
down to zero (which mutes), M flipped muted off while leaving the volume
at 0, so the player looked unmuted but stayed silent — in all three
legacy engine adapters.

Mirror the shared controls' ControlsVolume semantics with a per-adapter
LegacyMuteMemory: muting remembers the audible volume, and unmuting while
the volume sits at zero restores it, with the same 0.5 fallback when
nothing was remembered.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014h2cZi5DcFSbcmV7WgB6qB

---------

Co-authored-by: Claude <noreply@anthropic.com>

README.md

@@ -81,9 +81,9 @@ Press `?` or `Shift+/` in the workspace to open the in-app shortcuts list.
 | Global            | `Enter` in workspace search | Submit the current search                                  |
 | Navigation        | `Ctrl/Cmd+B`                | Toggle the live sidebar                                    |
 | Navigation        | `0-9`                       | Select an M3U channel by number                            |
-| Playback          | `Space` / `K`               | Play or pause embedded MPV playback in the desktop app     |
-| Playback          | `F`                         | Toggle embedded MPV fullscreen in the desktop app          |
-| Playback          | `ArrowLeft` / `ArrowRight`  | Seek embedded MPV playback by 5 seconds in the desktop app |
+| Playback          | `Space` / `K`               | Play or pause playback                                     |
+| Playback          | `F`                         | Toggle player fullscreen                                   |
+| Playback          | `ArrowLeft` / `ArrowRight`  | Seek VOD playback by 5 seconds                             |
 | Playback          | `ArrowUp` / `ArrowDown`     | Adjust volume by 5%                                        |
 | Playback          | `M`                         | Mute audio                                                 |
 | Dialogs and lists | `ArrowUp` / `ArrowDown`     | Move command palette selection                             |