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/docker-compose.test.yml

@@ -17,12 +17,12 @@ services:
       POSTGRES_PASSWORD: eventsnap_test
       POSTGRES_DB: eventsnap_test
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U eventsnap_test -d eventsnap_test"]
+      test: ['CMD-SHELL', 'pg_isready -U eventsnap_test -d eventsnap_test']
       interval: 3s
       timeout: 3s
       retries: 30
     ports:
-      - "55432:5432"               # exposed so the e2e harness can connect via pg for fixture setup
+      - '55432:5432' # exposed so the e2e harness can connect via pg for fixture setup
 
   app:
     build:
@@ -40,15 +40,15 @@ services:
       ADMIN_PASSWORD_HASH: $$2b$$04$$XKJJkNX6BOi6y3S42DA5JOWwk4oxc8DHPL6.MrPfJI2vpnccZjP32
       EVENT_SLUG: e2e-test-event
       EVENT_NAME: E2E Test Event
-      APP_PORT: "3000"
+      APP_PORT: '3000'
       MEDIA_PATH: /media
-      SESSION_EXPIRY_DAYS: "30"
-      EVENTSNAP_TEST_MODE: "1"     # ENABLES /admin/__truncate — never set in prod
+      SESSION_EXPIRY_DAYS: '30'
+      EVENTSNAP_TEST_MODE: '1' # ENABLES /admin/__truncate — never set in prod
       RUST_LOG: eventsnap_backend=info,tower_http=warn
     volumes:
       - media_data:/media
     expose:
-      - "3000"
+      - '3000'
 
   frontend:
     build:
@@ -57,11 +57,11 @@ services:
     depends_on:
       - app
     environment:
-      PORT: "3001"
-      HOST: "0.0.0.0"
-      ORIGIN: "http://localhost:3101"
+      PORT: '3001'
+      HOST: '0.0.0.0'
+      ORIGIN: 'http://localhost:3101'
     expose:
-      - "3001"
+      - '3001'
 
   caddy:
     image: caddy:2-alpine
@@ -71,7 +71,7 @@ services:
     volumes:
       - ./Caddyfile.test:/etc/caddy/Caddyfile:ro
     ports:
-      - "3101:3101"
+      - '3101:3101'
 
 volumes:
   media_data: