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/01-auth/logout-everywhere.spec.ts

@@ -5,14 +5,13 @@
  * revoked token must stop authenticating.
  */
 import { test, expect } from '../../fixtures/test';
-
-const BASE = process.env.E2E_FRONTEND_URL ?? 'http://localhost:3101';
+import { BASE } from '../../helpers/env';
 
 const ctx = (jwt: string) =>
   fetch(`${BASE}/api/v1/me/context`, { headers: { Authorization: `Bearer ${jwt}` } });
 
 test.describe('Auth — sign out everywhere', () => {
-  test('DELETE /sessions revokes ALL of the caller\'s sessions, not just the current one', async ({
+  test("DELETE /sessions revokes ALL of the caller's sessions, not just the current one", async ({
     api,
     guest,
     db,
@@ -41,7 +40,7 @@ test.describe('Auth — sign out everywhere', () => {
     expect(await db.countSessionsForUser(g.userId)).toBe(0);
   });
 
-  test('one user signing out everywhere does not touch another user\'s sessions', async ({
+  test("one user signing out everywhere does not touch another user's sessions", async ({
     guest,
   }) => {
     const a = await guest('SignsOut');