[FEATURE]: Desktop - Show session attention count on the Windows taskbar

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

When OpenCode Desktop runs on Windows and the app is not focused, show a persistent numeric attention badge on the Windows taskbar icon for sessions that finish or need user attention. The count should represent distinct sessions with the app's existing unread/session-attention state, similar to the small count badge used by ChatGPT.

The badge should clear a session when the user opens or views that session, while preserving the count for other sessions. This should use Electron's Windows taskbar API and remain Windows-only. It should not change the app layout or add broader system notification behavior.

Related context:

• #37120 covers missing desktop notification permission for approval requests.
• #24807 covers a TUI/Windows Terminal progress indicator.
• #30348 and #42811 cover related session unread/viewed state.
• #34684 added in-app unread state for pending questions.

This request is specifically for the native Windows taskbar indicator.