feat: Windows taskbar thumbnail toolbar media controls

[JoePittsy]

Summary

• Adds prev, play/pause, and next buttons to the Windows taskbar thumbnail preview for both the main window and mini player
• The play/pause icon swaps dynamically between play.png and pause.png based on live playback state from the playbackExtended WebSocket feed
• Buttons invoke wsSend directly — no IPC round-trip needed
• Assets in assets/taskbar/ are included in packaged builds via extraResources in electron-builder config
• No-op on non-Windows platforms (process.platform !== 'win32' guard)

Test plan

• Launch app on Windows, hover the taskbar icon — prev/play/next buttons appear in the thumbnail
• Start playback — button shows pause icon; pause — button shows play icon
• Each button triggers the correct Sonos command
• Open mini player — thumbnail toolbar also present and in sync
• Packaged build includes the PNG assets under resources/assets/taskbar/

🤖 Generated with Claude Code

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	71.38%	1931 / 2705
🔵	Statements	68.14%	2218 / 3255
🔵	Functions	62.22%	603 / 969
🔵	Branches	62.29%	2067 / 3318

File Coverage

No changed files found.

Generated in workflow #136 for commit fd3eb2a by the Vitest Coverage Report Action