test(infra): mountWithProviders helper + Vuetify CT sanity test

B2 of TEST-INFRA-001 (RFC-WS-FRONTEND-PRIMEVUE Amendment A-1).

- Add tests/playwright-ct/utils/mountWithProviders.ts: ergonomic
  wrapper around Playwright CT's mount() exposing buildMountArgs()
  and readNotificationState(). Documents the Vue Test Utils ↔
  Playwright CT API divergence (provider plugins must be wired in
  beforeMount, not at call time) and the Vuetify-temp lifecycle
  (replaced by PrimeVue in F3).
- Add tests/playwright-ct/components/SanityButtonHarness.vue: a
  v-btn harness with a click counter; lives in a .vue file so Vite
  bundles its CSS-side-effect imports for the browser context
  (Playwright CT runs the test orchestrator in Node and components
  in a Vite-bundled browser, unlike Vitest's single jsdom graph).
- Add tests/playwright-ct/components/sanity-vuetify.spec.ts: two
  tests proving (a) v-btn renders and propagates clicks, (b) the
  --v-theme-primary CSS variable resolves to a parseable RGB triplet.
- Update playwright/index.ts: import 'vuetify/styles' so the v-btn
  renders with its actual visual appearance (not unstyled). Required
  for B3's visual baselines.

3 component tests pass. 402 Vitest tests still pass unchanged.
Lint + typecheck clean on new files.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/app/playwright/index.ts

@@ -10,8 +10,13 @@ import { type ThemeDefinition, createVuetify } from 'vuetify'
 import * as components from 'vuetify/components' // eslint-disable-line no-restricted-imports
 import * as directives from 'vuetify/directives'
 
-// Plain-CSS token sheet — JSDOM evaluates :root custom properties from
-// this import so getComputedStyle(el).getPropertyValue('--tt-status-…')
+// Vuetify base styles — required for v-btn / v-chip / v-card etc. to
+// render with their actual visual appearance (not a default browser
+// look). Without this, Playwright visual baselines would capture
+// unstyled components. Removed in F3 alongside the Vuetify plugin.
+import 'vuetify/styles'
+
+// Plain-CSS token sheet — getComputedStyle(el).getPropertyValue('--tt-status-…')
 // resolves during component tests. Path resolved by the alias map in
 // playwright-ct.config.ts.
 import '@/styles/tokens/_timetable.css'