feat(mpris): pin preferred media source

[scottsteely]

Description

Add a minimal "pin media source" option so the bar media widget and OSD keep showing a preferred player (e.g. a music player) even when another source (e.g. YouTube in the browser) starts playing.

• MprisController: new pinnedIdentity property + setPinned(identity); _bestPlayingPlayer() prefers a playing pinned player (exact identity match, then case-insensitive substring fallback). Existing behavior is unchanged when no pin is set.
• Persistence via SessionData.pinnedPlayerIdentity (spec entry added so SessionData.set() accepts the key); empty string clears the pin.
• MediaDropdownOverlay (Media Players panel): each player row gets a theme-colored circle indicator on the right. Empty circle = no pin (initial state). Right-click a row to make it the default source (circle fills); right-click another row to move the pin; right-click the pinned row again to clear (back to empty). Left-click behavior unchanged (selects the player).

Type of change

• Bug fix (non-breaking change that fixes an issue)
• [ X] New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes existing behavior)
• Refactor / internal cleanup
• Documentation
• Other

Related issues

Screenshots / video

Checklist

• [X ] My code follows the conventions in CONTRIBUTING.md
• [X ] I have tested my changes locally
• [X ] New user-facing strings are wrapped in I18n.tr() with translator context, reusing existing terms where possible
• [ X] Go changes: ran make fmt, added/updated tests, make test passes, and go mod tidy is clean
• [X ] QML changes: ran make lint-qml with no new warnings
• I have opened a corresponding pull request in dlx-docs to document any new behaviors: https://github.com/AvengeMedia/DankLinux-Docs
  i'll do that last one next

[hthienloc]

/claude review

[claude]

Claude review

One minor issue; the feature itself looks sound and existing selection behavior is preserved when no pin is set.

• setPinned() imperatively assigns pinnedIdentity, destroying its binding to SessionData.pinnedPlayerIdentity (redundant, since SessionData.set() already propagates) — quickshell/Services/MprisController.qml:313

Checked: pin resolution order in _bestPlayingPlayer(), SessionData spec/persistence round-trip, delegate layout and right-click handling in MediaDropdownOverlay (no new user-facing strings, en.json/template.json untouched, no Go changes). Model: claude-opus-5.