Summary

This PR addresses all UI/UX refinements identified in Issue #700:

1. DataGridFilterBar Autocomplete Suggestions Theme Styling:

   • Replaced hardcoded hex colors (0xFF1E1E22 / 0xFFFFFFFF) with semantic theme tokens (cs.popover, cs.popoverForeground, and cs.border) so suggestions blend seamlessly across all dark and light themes.

2. File Export vs. Database Save Disambiguation:

   • Renamed query results toolbar export dropdown from Save ▾ to Export ▾ (and updated icon to Icons.file_download_outlined) across results_tab.dart and extension_table_toolbar.dart.
   • Keeps file export distinct from committing staged row changes to the database (Save Changes).

3. Filter Bar Close Action & Escape Shortcut:

   • Added onClose callback to DataGridFilterBar and wired it to ResultsTab.
   • Added close button (Close filter (Esc)) when empty.
   • Added key listener handling Escape to close the suggestions overlay, clear filter input, or collapse the filter bar.

4. DataGridStagingToolbar Hover States & Shortcut Tooltips:

   • Converted _ToolbarButton to stateful with smooth animated hover highlight.
   • Added tooltips with keyboard shortcuts for desktop UX:
     • Add Row: Ctrl+Insert / Cmd+N
     • Delete Row: Ctrl+Delete / Cmd+Backspace
     • Revert All: Ctrl+Z / Cmd+Z
     • Save Changes: Ctrl+S / Cmd+S commit hint

5. Automated Tests:

   • Added unit & widget test coverage in data_grid_filter_bar_test.dart and results_tab_test.dart.

Closes #700