style(frontend): Prettier config + one-time repo-wide format pass

Prettier 3 (pinned), printWidth 100, double quotes — chosen to match the existing
hand-formatting and minimise reflow. This commit is ONLY the mechanical format pass
(`prettier --write`) plus the config/ignore/scripts, isolated so it never pollutes a
feature diff. Verified behaviour-neutral: typecheck, eslint (0 errors), and vitest
all green before and after. See .git-blame-ignore-revs — `git blame` skips this sha.

frontend/e2e/feed.spec.ts

@@ -74,8 +74,6 @@ test.describe("feed", () => {
 
     // Switch the show filter → a different result set → back to the top.
     await page.getByTestId("feed-show-all").click();
-    await expect
-      .poll(() => page.getByRole("main").evaluate((el) => el.scrollTop))
-      .toBeLessThan(50);
+    await expect.poll(() => page.getByRole("main").evaluate((el) => el.scrollTop)).toBeLessThan(50);
   });
 });