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/layout/sidebar.tsx

@@ -84,7 +84,12 @@ export function Sidebar({
     }
     if (item.label === "Help") {
       return (
-        <button key={item.label} type="button" onClick={onHelpClick} className="w-full">
+        <button
+          key={item.label}
+          type="button"
+          onClick={onHelpClick}
+          className="w-full text-sidebar-foreground"
+        >
           {content}
         </button>
       );