Fix break reminder showing malformed 'Next:' output

[bearmug]

Summary

• Fix check_focus_break() in refresh-daemon.sh to handle missing/stale calendar data gracefully
• Check meeting is in future (> now_ms) not just non-zero to avoid past meetings
• Omit "Next:" suffix entirely when no upcoming meeting exists
• Fallback title to "meeting" if calendar entry has empty title

Before

☕ Break reminder ☕ 771m focus. Break? Next: in ∞m

After

☕ 771m focus. Break?

Or with a valid meeting:

☕ 771m focus. Break? Next: Standup in 45m

Test plan

• Existing test-focus.sh passes (19/19 tests)
• Manual test: Focus active, no calendar data → "Break?" with no "Next:"
• Manual test: Focus active, meeting in 30m → "Break? Next: <title> in 30m"
• Manual test: Focus active, meeting in 5m → No break reminder
• Manual test: Focus active, past meeting in calendar → "Break?" with no "Next:"