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/tests/playwright-ct/components/SanityButtonHarness.vue

@@ -0,0 +1,20 @@
+<script setup lang="ts">
+import { ref } from 'vue'
+
+const count = ref(0)
+function onClick() {
+  count.value += 1
+}
+</script>
+
+<template>
+  <div data-test="harness">
+    <VBtn
+      color="primary"
+      data-test="btn"
+      @click="onClick"
+    >
+      Clicks: {{ count }}
+    </VBtn>
+  </div>
+</template>