Add more breathing exercise patterns

[turqoisehex]

Summary

Screen Break has 3 breathing patterns (box, relaxing, energizing) in the `BREATHING_PATTERNS` dict at line ~341. Adding more patterns would give users variety during micro-pauses.

Acceptance Criteria

• Add 2-3 new patterns (e.g., 4-7-8 sleep breathing, alternate nostril timing, coherent breathing)
• Each pattern defines: name, phases list (with action + duration), and cycles count
• New patterns are randomly selected during breathing exercises
• Display correctly in the breathing exercise overlay

Relevant Code

• `BREATHING_PATTERNS` dict: `screen_break.py` line ~341
• Breathing overlay: search for `BreathingGuide` class (~line 450)
• Pattern selection during micro-pause: line ~2174

Notes

Adding a pattern is just adding an entry to the `BREATHING_PATTERNS` dict following the existing format. Each phase has an action string ("Inhale", "Hold", "Exhale") and a duration in seconds. The `BreathingGuide` class handles animation automatically.