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/10-flow-review/upload-lock-code.spec.ts

@@ -9,6 +9,7 @@ import { test, expect } from '../../fixtures/test';
 import { uploadRaw } from '../../helpers/upload-client';
 import { readFileSync } from 'node:fs';
 import { join } from 'node:path';
+import { BASE } from '../../helpers/env';
 
 const SAMPLE = () => readFileSync(join(process.cwd(), 'fixtures', 'media', 'sample.jpg'));
 
@@ -36,7 +37,6 @@ test.describe('Upload — locked event uses a distinct, reversible 403 (audit fi
   });
 
   test('released gallery → 403 uploads_locked (also reversible via reopen)', async ({ host }) => {
-    const BASE = process.env.E2E_FRONTEND_URL ?? 'http://localhost:3101';
     const rel = await fetch(`${BASE}/api/v1/host/gallery/release`, {
       method: 'POST',
       headers: { Authorization: `Bearer ${host.jwt}` },