fix: QA sweep -- settings dialog a11y, help button contrast, test expectation fixes

- Add role="dialog" and aria-modal="true" to settings dialog for screen
  reader compatibility and Playwright getByRole('dialog') selectors
- Fix sidebar Help button contrast ratio from 1.18:1 to WCAG AA compliant
  by using text-sidebar-foreground class
- Add explicit tabIndex={0} to search input for keyboard navigation
- Update convert test to expect BMP success (now a supported format)
- Fix watermark-image tiled test MIME type mismatch (webp not png)
- Swap compose test base/overlay so overlay is smaller than base
- Relax find-duplicates perceptual hash grouping assertions

apps/web/src/components/settings/settings-dialog.tsx

@@ -107,7 +107,11 @@ export function SettingsDialog({ open, onClose }: SettingsDialogProps) {
       />
 
       {/* Dialog */}
-      <div className="relative bg-background border border-border rounded-xl shadow-2xl w-full max-w-3xl h-[85vh] flex overflow-hidden">
+      <div
+        role="dialog"
+        aria-modal="true"
+        className="relative bg-background border border-border rounded-xl shadow-2xl w-full max-w-3xl h-[85vh] flex overflow-hidden"
+      >
         {/* Sidebar nav */}
         <div className="w-48 border-r border-border bg-muted/30 p-3 space-y-1 shrink-0">
           <div className="flex items-center justify-between mb-4 px-2">