feat(ui): morph ResultsTab idle/loading/error/grid (#342)

[ZhuchkaTriplesix]

Summary

• ResultsTab morphs idle → loading → error/status/affected → grid via QueryaFadeSlide.
• Mode keys are stable per mode (not per row/data), so grid refreshes and scroll virtualization do not re-trigger fades.
• Prefer FadeSlide over SwitchingBody so large grids are not keep-alive mounted under spinner/error.
• Expanded widget tests for modes, morph, motion-off snap, and row-update stability.

Phase B slice of #342 (results states). Remaining: dialog enter/exit, theme motion, split settle.

Linked issue

Refs #342

Checklist

• flutter analyze and flutter test pass locally
• Scope matches the linked issue only (no drive-by refactors)
• PR targets dev (not main, unless hotfix)

Test plan

• flutter test test/features/main_screen/results_tab_test.dart
• Manual: run SQL — idle → spinner → grid; force error — spinner → error text
• Manual: motion Off — mode swaps snap
• Manual: scroll a large result set — no per-row fade

Made with Cursor