feat: Add floating timer widget for background recording

Summary

When users switch tabs during a browser recording, the recording may be affected by browser throttling of background tabs. A floating timer widget (Picture-in-Picture style) would improve UX by:

• Showing recording status when tab is backgrounded
• Displaying timer and audio level indicator
• Being semi-transparent to not obstruct other work
• Allowing users to click to return to VaultNote

Problem

Browsers throttle setInterval and other timers in background tabs, which can cause:

• Recording duration timer to desync
• Potential early stop of recording
• Users unaware their recording is still active

Current Mitigation

Added a subtle hint text: "For best results, keep this tab visible while recording"

Proposed Solution

Implement a floating mini-player using one of:

1. Document Picture-in-Picture API (Chrome) - full mini-window
2. CSS position: fixed overlay - appears when tab loses focus
3. Browser notifications with live updates

Acceptance Criteria

• [ ] Floating widget appears when user switches tabs during recording
• [ ] Shows current recording time
• [ ] Shows audio level indicator (optional)
• [ ] Click to return to VaultNote tab
• [ ] Semi-transparent design
• [ ] Works across major browsers (Chrome, Firefox, Edge, Safari)

Related

• Issue #360: Tier-based quota system
• Bug #18: Recording stops early when tab is backgrounded (root cause: browser throttling)