[16.0][IMP] budget_report: improve performance and code quality

[n3n]

Summary

• Remove redundant children/lines from BudgetNode serialization to reduce JSON payload
• Batch N+1 department queries using read_group (2 queries instead of N per page load)
• Cache filter options on initial load instead of fetching on every data refresh
• Optimize visibleRows visibility check from O(n²) to O(n) with Map-based lookup
• Parallelize department RPC calls with Promise.all for improved responsiveness
• Clean up debug console.log statements
• Add loading spinner and empty state UX

Test plan

• Open "สรุปการใช้งบประมาณภาพรวม" report
• Verify loading spinner appears during data fetch
• Verify all expand/collapse functionality works
• Verify drill-down links open correct views
• Test department filter selection
• Check that no debug logs appear in browser console
• Test with empty data to verify empty state message