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

tests/e2e-docker/essential-tools.spec.ts

@@ -404,7 +404,7 @@ test.describe("Convert", () => {
     expect(body.processedSize).toBeGreaterThan(0);
   });
 
-  test("convert rejects invalid format", async ({ request }) => {
+  test("convert PNG to BMP", async ({ request }) => {
     const res = await request.post("/api/v1/tools/convert", {
       headers: { Authorization: `Bearer ${token}` },
       multipart: {
@@ -412,9 +412,10 @@ test.describe("Convert", () => {
         settings: JSON.stringify({ format: "bmp" }),
       },
     });
-    expect(res.ok()).toBe(false);
+    expect(res.ok()).toBe(true);
     const body = await res.json();
-    expect(body.error).toBeDefined();
+    expect(body.downloadUrl).toBeTruthy();
+    expect(body.processedSize).toBeGreaterThan(0);
   });
 });