Add unit, integration and e2e test suites

tests/{unit,integration,e2e}/, at the repo root (not under frontend/,
since e2e exercises the frontend and PocketBase together) — all
against real code rather than mocks:

- unit/: Vitest (nuxt environment) — pure functions in server/utils
  and shared/utils, plus components/composables via
  @nuxt/test-utils/runtime's mountSuspended.
- integration/: Vitest (node environment) — @nuxt/test-utils/e2e's
  setup() builds and runs the real Nitro server against a real
  ephemeral PocketBase instance, exercising the actual /content/* and
  /admin routes end to end.
- e2e/: Playwright, a real browser against the built app and another
  ephemeral PocketBase, covering the user-facing flows (home, blog
  archive, article detail incl. JSON-LD, category filter, 404s, the
  /admin redirect).

The Vitest/Playwright tooling (and node_modules) stays in frontend/,
the repo's only npm project; its configs just point at ../tests/.
Playwright resolves packages from the node_modules nearest each spec
file, so `npm run test:e2e` first symlinks tests/node_modules to
frontend/node_modules (pretest:e2e, idempotent, gitignored).

tests/support/pocketbase.ts is the shared piece behind integration
and e2e: it downloads the same pinned PocketBase binary
pocketbase/Dockerfile uses, starts it against the real
pocketbase/pb_migrations/ (not a copy), and seeds it from
tests/support/fixtures.ts — so these tests run against the exact
schema and API rules that ship to production, catching the class of
bug that only shows up when PocketBase actually enforces them (e.g. a
wrong filter/fields query string silently returning nothing, or too
much).

CLAUDE.md and docs/frontend.md document the new npm scripts and the
single-test commands for each layer.

frontend/package.json

@@ -9,7 +9,12 @@
     "preview": "nuxt preview",
     "postinstall": "nuxt prepare",
     "typecheck": "nuxt typecheck",
-    "lint": "eslint ."
+    "lint": "eslint .",
+    "test:unit": "vitest run --config vitest.unit.config.ts",
+    "test:integration": "vitest run --config vitest.integration.config.ts",
+    "pretest:e2e": "ln -sfn ../frontend/node_modules ../tests/node_modules",
+    "test:e2e": "playwright test",
+    "test": "npm run test:unit && npm run test:integration && npm run test:e2e"
   },
   "dependencies": {
     "marked": "^18.0.11",
@@ -20,8 +25,14 @@
   },
   "devDependencies": {
     "@nuxt/eslint": "^1.17.0",
+    "@nuxt/test-utils": "^4.3.2",
+    "@playwright/test": "^1.63.0",
+    "@vitest/coverage-v8": "^5.0.0",
+    "@vue/test-utils": "^2.5.0",
     "eslint": "^10.10.0",
+    "happy-dom": "^20.14.3",
     "typescript": "^5.9.3",
+    "vitest": "^5.0.0",
     "vue-tsc": "^3.3.11"
   }
 }