fix(mpris): match fully qualified D-Bus signal names in the watcher

[Rishabh672003]

godbus sets Signal.Name to interface.member — documented on the Signal type in v5.2.2
(conn.go: "The name member is given in 'interface.member' notation") and built as
Name: iface + "." + member. The D-Bus spec makes the INTERFACE header required for signals, so
the name is always qualified.

The switch in internal/plugins/mpris/watcher.go compared it against bare member names:

switch sig.Name {
case "NameOwnerChanged":
case "Seeked":
case "PropertiesChanged":
}

None of these can match, so handleNameOwnerChanged, handleSeeked and
handlePropertiesChanged never ran. This PR uses the qualified names:

• org.freedesktop.DBus.NameOwnerChanged
• org.mpris.MediaPlayer2.Player.Seeked
• org.freedesktop.DBus.Properties.PropertiesChanged

Why it appeared to mostly work

The plugin's fallback paths covered for the dead signal path:

• Players present at startup are listed directly, not via signals.
• While a player is playing, the position poller re-reads state every position_interval
  (2 s), so pauses and track changes arrived up to 2 s late.
• While all players are paused, the watchdog (watchdogInterval, 10 s) finds playback that
  started, so play-from-pause took up to ~10 s.
• Players appearing/disappearing were only picked up by the reconcile that runs when the
  phone requests the player list, since NameOwnerChanged was never handled.

With this change, updates are signal-driven again and the poller and watchdog go back to being
backups.

Testing

• A small program on godbus v5.2.2 against a live session bus printed exactly the three
  qualified names above for NameOwnerChanged, Seeked and PropertiesChanged.
• go test ./... (with and without -race), the integration suite, go vet and gofmt all pass.

AI-assisted.

[bethropolis]

Thank you for catching this, I think with this i can finally remove the watchdog.

[Rishabh672003]

yeah the watchdog isn't needed now, also @bethropolis i found this bcz someone made a vicinae extension (vicinaehq/extensions#398 its not merged as of now) for it, which is really cool. maybe you could mention it in the readme or somewhere, so people can get a useful implementation of this project

Any way really cool project, thanks for your work