feat(player): add VLC reuse-instance setting (#893)

VLC was unconditionally spawned per click — VLC's own single-instance
preference fails because the per-launch RC args defeat its D-Bus
forwarder. Mirror the existing MPV reuse pattern so users can opt in to
driving one tracked VLC via its RC interface (clear + add) instead of
opening a new window every stream.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Entire-Checkpoint: f957cd9849e0

docs/architecture/embedded-inline-playback.md

@@ -105,6 +105,7 @@ Current contract:
 - AppImage, deb/rpm, snap, macOS, and Windows keep the existing direct process spawn flow.
 - VLC keeps the current external-session flow in Flatpak, including the RC port used for progress polling.
 - MPV is intentionally reduced in Flatpak: the app does not reuse an existing MPV instance there and does not open the Unix socket bridge used for non-Flatpak progress polling.
+- VLC instance reuse is also gated off in Flatpak. Outside Flatpak the user can opt in via the "Reuse VLC instance" setting; the app then keeps a single tracked VLC process and drives subsequent stream loads through its RC interface (`clear` + `add <url> :http-*`) instead of spawning a new window per click.
 
 This keeps non-Flatpak behavior unchanged while allowing Flatpak builds to open host-installed external players.