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/sse-eviction.spec.ts

@@ -7,8 +7,7 @@
 import { test, expect } from '../../fixtures/test';
 import { seedUpload } from '../../helpers/seed';
 import { SseListener } from '../../helpers/sse-listener';
-
-const BASE = process.env.E2E_FRONTEND_URL ?? 'http://localhost:3101';
+import { BASE } from '../../helpers/env';
 
 test.describe('Host — live SSE eviction (H3)', () => {
   test('self-deleting an upload broadcasts upload-deleted', async ({ guest }) => {
@@ -24,17 +23,10 @@ test.describe('Host — live SSE eviction (H3)', () => {
     });
     expect(res.status).toBe(204);
 
-    await sse.waitForEvent(
-      'upload-deleted',
-      (e) => e.data.upload_id === uploadId
-    );
+    await sse.waitForEvent('upload-deleted', (e) => e.data.upload_id === uploadId);
   });
 
-  test('banning a user broadcasts user-hidden', async ({
-    api,
-    host,
-    guest,
-  }) => {
+  test('banning a user broadcasts user-hidden', async ({ api, host, guest }) => {
     const target = await guest('HideTarget');
     await seedUpload(target.jwt, { caption: 'should vanish' });
 
@@ -43,10 +35,7 @@ test.describe('Host — live SSE eviction (H3)', () => {
 
     await api.banUser(host.jwt, target.userId);
 
-    await sse.waitForEvent(
-      'user-hidden',
-      (e) => e.data.user_id === target.userId
-    );
+    await sse.waitForEvent('user-hidden', (e) => e.data.user_id === target.userId);
   });
 
   // Frontend regression: the broadcasts above are inert if the client never