fix(ui): SQL toasts, chrome scale, Redis/Mongo empty states

[ZhuchkaTriplesix]

Summary

• ui(sql): surface open/save file errors in workspaces #452 SQL workspace open/save failures → error toast
• ui(shell): scale window title bar height with UI scale #453 Title bar height follows UI scale
• ui(redis): surface TYPE/TTL batch failure on keys view #454 Redis keys TYPE/TTL failure → non-blocking banner
• ui(stats): empty-state with Retry for Redis/Mongo blank payload #455 Redis/Mongo blank stats → empty-state + Retry
• ui(redis): mounted guard after await in key editor saves #456 Redis key editor post-await mounted guards
• ui(results): ResultsTab isLoading must not hide errorMessage #457 ResultsTab documents loading→error order + test

Closes #452
Closes #453
Closes #454
Closes #455
Closes #456
Closes #457

Test plan

• flutter test test/features/main_screen/results_tab_test.dart
• Fail open/save SQL file — toast appears
• High UI scale — title bar not clipped