feat(media): improve multi-player session selection

[NicodeSS]

What

Improve media session selection when multiple SMTC-enabled players are available. The existing player-name button remains the entry point: right-click it to choose a player.

Why

Choosing a player for a quick action should not create a lasting preference. The flyout and taskbar should also stay aligned with the session being controlled as playback and Windows' current session change.

Changes

• Keep manual selection in memory instead of saving a player pin. Return to Windows' current session when the selected session becomes unavailable.
• Add a "Follow system" menu item that shows the current target, alongside player icons, track information, playback status, and a right-click hint.
• Add a switch for manual player selection and an option to resume following Windows when another player starts playing. Turning off manual selection clears the current choice immediately.
• Refresh the flyout and taskbar when Windows changes its current session, and ignore updates from unrelated sessions.

Testing

• dotnet build FluentFlyoutWPF/FluentFlyout.csproj -c Debug --no-restore -v quiet — passed.
• dotnet format FluentFlyout.sln --verify-no-changes --no-restore — passed.
• Windows preview tested; the expected selection behavior was confirmed.