feat: comprehensive trigger node system with Schedule Trigger implementation

[lyzno1]

Related #24001
Related #23981

Summary

Implements the foundational architecture for trigger-based workflow nodes, introducing three new trigger types and a conditional Start tab system.

Changes:

• Add 3 trigger node types: Schedule, Webhook, Plugin
• Implement conditional Start tab in block selector (shows only trigger nodes)
• Add workflow entry point abstraction with trigger priority
• Integrate export functionality into More Actions dropdown

Current Limitations

• Start node exclusion: Due to code constraints, the traditional Start node (User Input) cannot appear in the Start tab currently
• No uniqueness constraint: All entry nodes (triggers + start) lack global uniqueness enforcement - multiple entry nodes can exist simultaneously
• Placeholder implementations: Schedule/Webhook/Plugin panels show placeholder content only
• Missing Test Run: Preview button not yet replaced with Test Run dropdown

Next Steps

• Implement Schedule Trigger configuration panel
• Add Test Run functionality
• Enforce entry node uniqueness constraints
• Include User Input in Start tab

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[lyzno1]

Schedule Trigger Implementation Status

✅ Completed Core Features

• Schedule Trigger Node: Complete implementation with dual-mode configuration (Visual Picker + Cron Expression)
• Visual Picker Configuration: All 5 frequency types implemented
  • Hourly: Start time + recur every (Hours/Minutes toggle)
  • Daily: Time picker with Now/OK buttons (white/blue styling)
  • Weekly: Time picker + weekday selector (7-button grid)
  • Monthly: Time picker + days selector (1-31 grid + Last day with tooltip)
  • Once: Date-time picker for one-time execution
• Cron Expression Mode: Full cron parser with validation and preview
• Next Execution Display: Shows next 5 execution times in node and panel
• Execute Now Button: Horizontal button for immediate workflow triggering
• Timezone Handling: Unified system timezone across all calculations (46 test cases, 100% coverage)

✅ Architecture & UI Implementation

• Start Tab System: Conditional tab showing trigger nodes only
• Node Panel Structure: Settings tab implemented (Last Run tab placeholder ready)
• Responsive Design: Grid-based layouts, dark mode support, proper spacing alignment
• International Support: Complete translations for Chinese (zh-Hans) and Japanese (ja-JP)

✅ Technical Quality

• Comprehensive Testing: 46 test cases covering all execution scenarios
• Smart Algorithm: Monthly last-day handling, timezone consistency, duplicate prevention
• Component Architecture: Reusable DateTimePicker, FrequencySelector, ExecutionTimeCalculator
• Performance: Efficient state management and render optimization

🔄 Remaining Items (Per Design Doc)

• Last Run tab implementation (execution history display)
• Test Run functionality enhancement (replacing Preview button)
• Webhook/Plugin trigger configuration panels
• Trigger variable integration into workflow variable system

The Schedule Trigger implementation fully satisfies the design requirements from the frontend specification document and provides a solid foundation for the complete trigger system.