feat(app): add PR card and desktop workflow upgrades

[DLHTX]

Summary

• add current-branch GitHub PR lookup and render a header PR status card with one-click open
• add term UI rendering, window activity tracking, and notification/audio plumbing across the app
• switch worktree untracked file handling to symlinks and wire the supporting backend and UI updates

Test plan

• Run cargo test -p oxmux-server github
• Run cargo build --features agent
• Run npm run build in web/
• Verify /api/health on port 1235 and /api/github/current-pr return valid responses
• Verify the header PR card, worktree symlink flow, and term UI rendering behavior in the app