feat(media): expose session switching in both flyout layouts

[NicodeSS]

What

Expose the existing media-session switcher directly in the media flyout. The original player button remains when only one eligible session is available.

Why

The current switcher is only discoverable through a right-click on the player button in the normal layout. Compact layout has no equivalent entry point. A visible selector makes switching available in both layouts without changing the underlying session-selection policy.

Changes

• Show a SplitButton in the normal layout when at least two eligible sessions exist; its main area still opens the player, while the arrow opens the session menu.
• Add a compact dropdown selector when multiple sessions exist in compact layout or when the player name is hidden.
• Reuse the existing Automatic and pinned-session menu behavior, including its persisted PinnedSessionId setting.
• Update visibility and sizing as sessions appear or disappear, and keep the flyout open while the menu is active.

This PR changes the selection UI only. It does not change how the current media session is chosen or stored.

Maintainer note

You mentioned developing a SplitButton implementation in your own branch. I could not locate that implementation in the public branches or PRs I checked, so this is a working UI proposal for now. I am happy to adapt this PR to your version, or replace this SplitButton with it once it is available.

Testing

• dotnet build FluentFlyoutWPF/FluentFlyout.csproj -c Debug --no-restore -v quiet — passed (0 errors; 54 warnings also present on the unmodified base).
• dotnet format FluentFlyout.sln --verify-no-changes --no-restore --verbosity quiet — passed.
• Local preview launched and manually checked.

[unchihugo]

Hi @NicodeSS, thanks for the PR. I will take a look at it later today. I've developed a split button solution in a branch, but I might not have pushed it to the repo yet - sorry about that.