fix(mpris): fall back to next non-ignored player and prefer playing players

[BlueManCZ]

When using playerctld, the MPRIS module had several issues with player selection:

• Ignored players caused widget to disappear — If the most recently active player was in
  ignored-players, getPlayerInfo() returned std::nullopt with no fallback, hiding the widget
  entirely even when other valid players were available.
• Non-playing players took priority — Opening a new player (e.g. Spotify) would immediately
  take over the widget even if another player (e.g. Tidal) was actively playing, because playerctld
  orders by most recently active rather than playback state.
• Widget flickered on player changes — onPlayerStop hid the widget before triggering the
  update, causing a visible flash before update() re-showed it with the correct player.

Changes

• Iterate through playerctld's activity-ordered player list, skipping ignored players, and prefer
  a playing player over a paused/stopped one
• When the selected player differs from the playerctld proxy's top player, create a direct
  PlayerctlPlayer connection for correct metadata
• Remove premature event_box_.set_visible(false) from onPlayerStop — update() already
  handles visibility

Test plan

• Configure ignored-players with a browser (e.g. ["firefox"]), have another player paused
  — widget should show the paused player with correct metadata when browser plays media
• Play Tidal, then open Spotify without playing — widget should stay on Tidal
• Play Spotify while Tidal is paused — widget should switch to Spotify
• Pause all players — widget displays last active player as paused
• With all players ignored — widget should hide
• Player changes should not cause visible flicker

Fixes: #1954, fixes: #3879, fixes: #4610