ui(results): UI kit in results toolbar, charts and groupings (segmented control, icon buttons, export, empty states) #1162

Problem

The results area is the least consistent part of the UI:

• material.SegmentedButton for Grid / Groupings / Charts (results_tab.dart) and Bar / Line / Pie (quick_chart_view.dart:179), styled by the Material theme, not by workbench tokens.
• material.IconButton for chart export (quick_chart_view.dart:221, :227), the quick-filter toggle in the results toolbar and the groupings view (data_grid_groupings_view.dart:245, :264), instead of QueryaIconButton.
• Chart export is two unlabelled icons; the SVG one is polyline, which says nothing.
• Empty states are raw text: No numeric column to chart, No data, No data available for grouping.

Scope

• Segmented controls: one shared widget on top of QueryaTabStrip tokens (or QueryaTabStrip itself), used by results view mode, chart type and the MQL console Grid / JSON.
• QueryaIconButton everywhere in the results toolbar, charts and groupings.
• Chart Export ▾ (QueryaActionMenu): PNG, SVG, Copy image.
• QueryaEmptyState with a hint: Charts need a numeric column. Cast it in SQL, e.g. count(*)::int.
• A guard test that forbids material.SegmentedButton and bare material.IconButton under lib/features/ (like the existing ones in test/guards/).

Acceptance

• No material.SegmentedButton / material.IconButton left in results, charts, groupings, MQL console.
• New guard test passes.