* fix(overlay): reload imported mpv keys after mpv connects
The overlay fetched mpv's input-bindings before SubMiner connected to mpv,
cached an empty list, and never refetched because the connect-time refresh
only broadcast session bindings when their compiled signature changed. Keys
from input.conf and mpv defaults (9/0 volume, m mute) then did nothing while
the overlay had focus, most often in mpv.backend x11 mode.
Main now emits mpv-input-bindings:changed whenever the discovered mpv key
set or client changes, and the renderer refreshes its imported keys on it.
* fix(overlay): subscribe to mpv key changes before initial discovery
The renderer registered the mpv-input-bindings:changed listener only after awaiting setupMpvInputForwarding and several other startup calls. If mpv connected in that window, the event was dropped and the imported mpv keys stayed empty.
