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/04-host/event-lock.spec.ts

@@ -5,13 +5,16 @@
  * for a guest who's already viewing the feed.
  */
 import { test, expect } from '../../fixtures/test';
+import { BASE } from '../../helpers/env';
 
 test.describe('Host — event lock', () => {
-  test('closing the event via API sets uploads_locked_at; opening clears it', async ({ host, api }) => {
+  test('closing the event via API sets uploads_locked_at; opening clears it', async ({
+    host,
+    api,
+  }) => {
     // The frontend doesn't (yet) render a per-guest "uploads locked" banner on
     // the feed — that's the journey §9 banner, currently a UX gap. We assert
     // the API + DB contract here and leave the banner check for once it ships.
-    const BASE = process.env.E2E_FRONTEND_URL ?? 'http://localhost:3101';
 
     await api.closeEvent(host.jwt);
     const evRes = await fetch(`${BASE}/api/v1/host/event`, {
@@ -38,8 +41,11 @@ test.describe('Host — event lock', () => {
   // event (USER_JOURNEYS §9.3, FEATURES capability matrix). Only new uploads are
   // rejected. (An earlier revision froze social interaction too; that contradicted
   // the documented behavior and was reverted.)
-  test('a closed event still allows likes and comments, but blocks new uploads', async ({ api, host, guest }) => {
-    const BASE = process.env.E2E_FRONTEND_URL ?? 'http://localhost:3101';
+  test('a closed event still allows likes and comments, but blocks new uploads', async ({
+    api,
+    host,
+    guest,
+  }) => {
     const g = await guest('SocialLocked');
 
     // Upload while still open so there's a target to interact with.