WCAG 2.2 AA accessibility audit for export preview panel

Run the full accessibility test suite as a dedicated test file (export_preview_panel_a11y_test.dart) so it can be targeted separately in CI. For contrast ratio checks, implement a helper function contrastRatio(Color foreground, Color background) using the WCAG relative luminance formula — this avoids a third-party dependency. Fix any violations found during the audit in-place before marking this task complete; the audit is not a report task but a fix-and-verify task. Pay special attention to the skeleton loader: shimmer animations can produce animated Semantics nodes that confuse screen readers — suppress Semantics on skeleton rows with ExcludeSemantics.

For the live-region announcement on state transition, use a SemanticsService.announce() call within the BlocListener in ExportPreviewPanel rather than relying solely on Semantics(liveRegion: true), as the latter may not fire reliably on all platforms for dynamic content updates.

Automated tests using flutter_test: (1) Semantics label completeness — use tester.getSemantics on every named finder and assert label is non-empty; (2) Tap target size — use tester.getSize on all GestureDetector and InkWell widgets and assert >= Size(44, 44); (3) Live region — pump state transitions and verify SemanticsNode.liveRegion == true for the warning section and the panel root; (4) Accessibility guidelines — use expectLater(tester, meetsGuideline(androidTapTargetGuideline)) and meetsGuideline(iOSTapTargetGuideline) after each state render; (5) Focus traversal — use FocusTraversalPolicy traversal simulation to verify focus visits nodes in the documented order. Manual checklist: activate VoiceOver on iOS device, open ExportPreviewPanel, verify each section is announced correctly in all three states (loading, loaded with warnings, error). Document results in a test report artifact.