refactor: optimize scene prompt formatting and selection sync logic

[yhjun1026]

Summary

• Extract scene separator as variable for better code readability in agent_chat.py
• Improve scene selection synchronization logic in tab-scenes.tsx to avoid unnecessary state updates
• Add smart detection for server/client scene synchronization to prevent race conditions

Changes

• agent_chat.py: Refactored scene prompt formatting by extracting \n\n---\n\n as a named variable
• tab-scenes.tsx: Enhanced scene selection sync logic with conditions to check for new scenes from server and avoid overriding local unsynchronized changes

Test Plan

• Verified scene creation flow works correctly
• Tested scene selection persistence across app reloads
• Confirmed no state update loops in scene management

[tptpp]

R+