test: expand coverage across all layers -- 1,268 new tests, fix replace-color div-by-zero

14-agent parallel test expansion covering integration, unit, E2E, E2E-Docker,
cross-format matrix, adversarial, GUI navigation/tools/settings/visual/a11y/perf.

- Integration: expand 23 tool test files with HEIC, stress, batch, edge cases
- Unit: close coverage gaps in image-engine, stores, lib (metadata, auto-enhance,
  connection-store, lazy-with-retry, collage/file-store HEIC preview)
- AI bridge: 141 new tests for dispatcher buffering, crash recovery, OOM/segfault
- Cross-format: 794 parameterized tests (16 formats x 12 tools + no-crash matrix)
- Adversarial: memory stress (50x large file), zero-byte, corrupted headers, unicode
- E2E-Docker: expand 8 spec files with dimension verification, pipeline chains
- GUI E2E: tool UI settings/interactions for all 47 tools, remove all test.skip,
  RBAC per-role verification, visual screenshot naming, cross-browser smoke tests,
  a11y ARIA/focus/contrast, performance budgets, 15-tool stability test
- Fix: replace-color.ts tolerance=0 caused division-by-zero producing NaN pixels

Total: 8,958 tests passing across 202 files. Zero failures, zero skips.

tests/e2e/gui-file-carry.spec.ts

@@ -107,6 +107,30 @@ test.describe("Cross-tool file carrying", () => {
     await expect(page.getByRole("link", { name: /download/i })).not.toBeVisible();
   });
 
+  test("upload on resize, browser back returns to previous page", async ({
+    loggedInPage: page,
+  }) => {
+    // Start on the home page
+    await page.goto("/");
+    await page.waitForLoadState("networkidle");
+
+    // Navigate to resize
+    await page.goto("/resize");
+    await page.waitForLoadState("networkidle");
+
+    // Upload a file on resize
+    await uploadTestImage(page);
+
+    // Confirm the file is loaded
+    await expect(page.getByText(/test-image/i).first()).toBeVisible();
+
+    // Press browser back
+    await page.goBack();
+
+    // Should return to the home page
+    await expect(page).toHaveURL("/");
+  });
+
   test("file carry works only via Quick Actions, not via sidebar navigation", async ({
     loggedInPage: page,
   }) => {