chore(e2e): add ESLint + Prettier; fix real findings; dedupe BASE

The Playwright suite had no linter and no formatter — only tsc. Add flat-config ESLint
(typescript-eslint, type-aware) and Prettier (2-space, matching the suite's style).

Rules keep the ones that catch real TEST bugs and drop the noise:
  - no-floating-promises KEPT — an un-awaited request/assertion can let a test end before it runs,
    passing vacuously. It caught one: the SSE reader loop in sse-listener is now explicitly `void`.
  - no-unused-vars KEPT — caught three dead bindings (an unused adminToken fixture arg, an unused
    `api` arg, an unused JPEG_MAGIC import), all removed.
  - no-explicit-any OFF — all test code; `any` is the honest type for an untyped res.json() body or
    a page.evaluate() return.
  - no-empty-pattern OFF — Playwright's dependency-free fixtures are `async ({}, use) => {}`.

Refactor: `const BASE = process.env.E2E_FRONTEND_URL ?? '...'` was redeclared verbatim in 23
files — extracted to helpers/env.ts and imported, so a port/scheme change is one edit not a sweep.

Then `prettier --write`. Verified: eslint clean, tsc clean, prettier clean, desktop suite 210
passed / 1 skipped. (One mobile spec flaked once under retries:0 — a pre-existing cross-test
reflow-timing vector from the flakiness audit, not this change: the each-key edit is stable across
16 isolated runs and a clean full mobile re-run.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

e2e/specs/09-mobile/planned-gestures.spec.ts

@@ -19,7 +19,11 @@ import { join } from 'node:path';
 const SAMPLE_JPG = join(process.cwd(), 'fixtures', 'media', 'sample.jpg');
 
 test.describe('Mobile — planned gestures (fixme until shipped)', () => {
-  test.fixme('swipe left in lightbox navigates to next filtered item', async ({ page, guest, signIn }) => {
+  test.fixme('swipe left in lightbox navigates to next filtered item', async ({
+    page,
+    guest,
+    signIn,
+  }) => {
     const author = await guest('SwipeAuthor');
     // Seed two uploads so there's a "next" to navigate to.
     for (const cap of ['First', 'Second']) {
@@ -35,7 +39,10 @@ test.describe('Mobile — planned gestures (fixme until shipped)', () => {
     await page.goto('/feed');
 
     // Open the lightbox on the first card.
-    const firstImage = page.locator('article').filter({ hasText: 'First' }).getByRole('button', { name: 'Bild vergrößern' });
+    const firstImage = page
+      .locator('article')
+      .filter({ hasText: 'First' })
+      .getByRole('button', { name: 'Bild vergrößern' });
     await firstImage.click();
     const lightbox = page.getByRole('dialog');
     await expect(lightbox).toBeVisible();
@@ -96,11 +103,7 @@ test.describe('Mobile — planned gestures (fixme until shipped)', () => {
     const box = await page.locator('body').boundingBox();
     if (!box) throw new Error('body not visible');
     // Pull down from the top of the viewport.
-    await swipe(
-      page,
-      { x: box.x + box.width / 2, y: 20 },
-      { x: box.x + box.width / 2, y: 200 }
-    );
+    await swipe(page, { x: box.x + box.width / 2, y: 20 }, { x: box.x + box.width / 2, y: 200 });
 
     await page.waitForTimeout(1_000);
     expect(deltaCalled).toBe(true);