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/gestures-doubletap.spec.ts

@@ -26,12 +26,18 @@ async function seedUpload(token: string, caption = 'Doubletap fixture'): Promise
     contentType: 'image/jpeg',
     caption,
   });
-  if (res.status !== 201) throw new Error(`Upload seed failed (${res.status}): ${await res.text()}`);
+  if (res.status !== 201)
+    throw new Error(`Upload seed failed (${res.status}): ${await res.text()}`);
   return (await res.json()) as { id: string };
 }
 
 test.describe('Mobile — double-tap gesture', () => {
-  test('double-tap on a feed card image button registers a like', async ({ api, page, guest, signIn }) => {
+  test('double-tap on a feed card image button registers a like', async ({
+    api,
+    page,
+    guest,
+    signIn,
+  }) => {
     const author = await guest('DtAuthor');
     const liker = await guest('DtLiker');
     const { id: uploadId } = await seedUpload(author.jwt, 'Double-tap me');
@@ -40,7 +46,8 @@ test.describe('Mobile — double-tap gesture', () => {
     await page.goto('/feed');
 
     // Locate the image button inside the card. The aria-label is "Bild vergrößern".
-    const imageButton = page.locator('article')
+    const imageButton = page
+      .locator('article')
       .filter({ hasText: author.displayName })
       .first()
       .getByRole('button', { name: 'Bild vergrößern' });
@@ -50,16 +57,26 @@ test.describe('Mobile — double-tap gesture', () => {
 
     // Wait for the optimistic increment OR the SSE-confirmed count. We assert via the
     // API to avoid coupling to specific DOM markup for the like badge.
-    await expect.poll(async () => {
-      const feed = await api.getFeed(liker.jwt);
-      // Backend returns { uploads: [...], next_cursor }.
-      const list: any[] = feed.uploads ?? feed.items ?? feed;
-      const row = Array.isArray(list) ? list.find((u: any) => u.id === uploadId) : undefined;
-      return row?.like_count ?? row?.likes ?? 0;
-    }, { timeout: 5_000 }).toBeGreaterThanOrEqual(1);
+    await expect
+      .poll(
+        async () => {
+          const feed = await api.getFeed(liker.jwt);
+          // Backend returns { uploads: [...], next_cursor }.
+          const list: any[] = feed.uploads ?? feed.items ?? feed;
+          const row = Array.isArray(list) ? list.find((u: any) => u.id === uploadId) : undefined;
+          return row?.like_count ?? row?.likes ?? 0;
+        },
+        { timeout: 5_000 }
+      )
+      .toBeGreaterThanOrEqual(1);
   });
 
-  test('double-tap inside the lightbox triggers the heart-burst (like recorded)', async ({ api, page, guest, signIn }) => {
+  test('double-tap inside the lightbox triggers the heart-burst (like recorded)', async ({
+    api,
+    page,
+    guest,
+    signIn,
+  }) => {
     const author = await guest('LbAuthor');
     const liker = await guest('LbLiker');
     const { id: uploadId } = await seedUpload(author.jwt, 'Lightbox heart');
@@ -68,7 +85,8 @@ test.describe('Mobile — double-tap gesture', () => {
     await page.goto('/feed');
 
     // Open the lightbox by clicking the image button.
-    const imageButton = page.locator('article')
+    const imageButton = page
+      .locator('article')
       .filter({ hasText: author.displayName })
       .first()
       .getByRole('button', { name: 'Bild vergrößern' });
@@ -88,12 +106,17 @@ test.describe('Mobile — double-tap gesture', () => {
 
     await doubleTap(page, media);
 
-    await expect.poll(async () => {
-      const feed = await api.getFeed(liker.jwt);
-      // Backend returns { uploads: [...], next_cursor }.
-      const list: any[] = feed.uploads ?? feed.items ?? feed;
-      const row = Array.isArray(list) ? list.find((u: any) => u.id === uploadId) : undefined;
-      return row?.like_count ?? row?.likes ?? 0;
-    }, { timeout: 5_000 }).toBeGreaterThanOrEqual(1);
+    await expect
+      .poll(
+        async () => {
+          const feed = await api.getFeed(liker.jwt);
+          // Backend returns { uploads: [...], next_cursor }.
+          const list: any[] = feed.uploads ?? feed.items ?? feed;
+          const row = Array.isArray(list) ? list.find((u: any) => u.id === uploadId) : undefined;
+          return row?.like_count ?? row?.likes ?? 0;
+        },
+        { timeout: 5_000 }
+      )
+      .toBeGreaterThanOrEqual(1);
   });
 });