Replace notification options with multi-device selector

[thetic]

Summary

• Remove notification group input
• Support selecting zero, one, or multiple mobile devices
• Use empty list default which works correctly for device selectors
• Fall back to persistent notifications when no devices selected

Fixes the "Unknown device selected" error that appeared in existing automations.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.35%. Comparing base (193cf94) to head (2898c18).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
- Coverage   98.68%   98.35%   -0.33%     
==========================================
  Files           6        6              
  Lines         304      304              
==========================================
- Hits          300      299       -1     
- Misses          4        5       +1     

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