feat: implement multi-select monthly trigger schedule

[lyzno1]

Summary

Enhanced monthly trigger schedule to support multi-day selection within a month while maintaining 100% backward compatibility with existing single-day configurations.

Key Changes

Data Structure Optimization

• Unified to single monthly_days: (number | 'last')[] field, removing dual-field complexity
• Follows existing weekdays: string[] pattern for API consistency
• Single selection: [15], Multi-selection: [1, 15, 30]

Component Enhancement

• MonthlyDaysSelector: Toggle-based multi-select UI with visual state feedback
• Execution Calculator: Enhanced algorithm supporting multiple days with deduplication and chronological sorting
• Validation: Robust array validation with range checking (1-31) and 'last' day support
• Panel Integration: Seamless array-based data binding with default fallbacks

Backward Compatibility

• Existing single-day configurations automatically work as single-element arrays
• No data migration required - handled at code level
• UI behavior consistent for single selections

Technical Implementation

Enhanced Features

• Multi-day Execution: Execute on multiple days within the same month
• Last Day Support: Special 'last' value handles month-end variations (28-31 days)
• Smart Scheduling: Automatically skips invalid days (e.g., Feb 30th)
• Leap Year Handling: Correct February 29th handling for leap/non-leap years
• Time Sorting: Chronological execution order within and across months

Edge Cases Handled

• Empty selection arrays → fallback to day 1
• Invalid days (>31) → validation rejection
• Duplicate selections → automatic deduplication
• Months with fewer days → smart day adjustment
• Past execution times → automatic next-month scheduling

Test Coverage (43 test cases)

Component Tests (17 cases)

• Single/Multi Selection: Toggle behavior, visual states, mixed selections
• Structure Validation: 31-day grid, 'last' button, proper labeling
• Accessibility: Keyboard navigation, visual distinction, tooltip support
• Interaction Logic: Add/remove selection, empty array handling

Validation Tests (9 cases)

• Valid Configurations: Single day, multiple days, 'last' day, mixed selections
• Invalid Configurations: Empty arrays, out-of-range days (>31), type validation
• Integration: Time format validation, large array handling (all 31 days)

Execution Calculator Tests (17 cases)

• Multi-select Logic: Same-month multiple executions, chronological sorting
• Calendar Handling: Leap year February, month-end variations, invalid day skipping
• Time Processing: AM/PM conversion, 12-hour format handling, timezone consistency
• Edge Cases: Duplicate day handling, search limits, past-time scheduling

Validation Logic Enhancement

Updated validateMonthlyConfig() to handle array-based monthly_days with comprehensive checks:

• Non-empty array requirement
• Range validation (1-31 + 'last')
• Type safety for mixed number/'last' arrays
• Integration with existing time validation

Files Modified

• types.ts: Unified data structure
• monthly-days-selector.tsx: Multi-select UI component
• default.ts: Enhanced validation logic
• execution-time-calculator.ts: Multi-day execution algorithm
• panel.tsx: Array-based integration

Test Files Added

• monthly-days-selector.test.tsx: Component behavior and accessibility
• monthly-validation.test.ts: Validation logic and edge cases
• monthly-multiselect.test.ts: Execution time calculation and calendar handling

All tests pass with high coverage: 100% component statements, 95% component branches.