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/02-upload/rate-limit.spec.ts

@@ -14,7 +14,11 @@ const SAMPLE_JPG = join(process.cwd(), 'fixtures', 'media', 'sample.jpg');
 const SAMPLE_BYTES = readFileSync(SAMPLE_JPG);
 
 test.describe('Upload — rate limit', () => {
-  test('4th upload in one hour returns 429 with Retry-After', async ({ api, adminToken, guest }) => {
+  test('4th upload in one hour returns 429 with Retry-After', async ({
+    api,
+    adminToken,
+    guest,
+  }) => {
     // Enable inside the test body, AFTER all auto-fixtures (truncate) have run,
     // so the config can't be reset out from under us by the ordering of hooks
     // vs fixtures.
@@ -53,7 +57,11 @@ test.describe('Upload — rate limit', () => {
     expect(limited.headers.get('retry-after')).toBeTruthy();
   });
 
-  test('flipping upload_rate_enabled off bypasses the limit', async ({ api, adminToken, guest }) => {
+  test('flipping upload_rate_enabled off bypasses the limit', async ({
+    api,
+    adminToken,
+    guest,
+  }) => {
     await api.patchConfig(adminToken, { upload_rate_enabled: 'false' });
 
     const h = await guest('NoQuota');