feat: add per-space timezone settings and improve booking management

- Add timezone configuration per space with fallback to system default
- Implement timezone-aware datetime display and editing across frontend
- Add migration for per_space_settings table
- Update booking service to handle timezone conversions properly
- Improve .gitignore to exclude build artifacts
- Add comprehensive testing documentation

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/migrations/004_add_per_space_settings.sql

@@ -0,0 +1,7 @@
+-- Add per-space scheduling settings (NULL = use global default)
+-- Note: SQLite doesn't support COMMENT ON COLUMN, but NULL values mean "use global default"
+
+ALTER TABLE spaces ADD COLUMN working_hours_start INTEGER;
+ALTER TABLE spaces ADD COLUMN working_hours_end INTEGER;
+ALTER TABLE spaces ADD COLUMN min_duration_minutes INTEGER;
+ALTER TABLE spaces ADD COLUMN max_duration_minutes INTEGER;