feat: add CI test workflow and improve i18n validation (#580)

- Add GitHub Actions workflow to run backend (pytest) and frontend
  (vitest) tests on PRs and pushes to main
- Replace hardcoded locale parity tests with dynamic discovery from
  the resources registry, so new languages are tested automatically
- Add unused key detection test that scans source files for i18n
  key references, with optional chaining normalization
- Remove 149 genuinely unused translation keys from all 7 locale files
- Add missing keys to it-IT (7) and ru-RU (5) with English fallback

.github/workflows/test.yml

@@ -0,0 +1,59 @@
+name: Tests
+
+on:
+  pull_request:
+    branches: [main]
+  push:
+    branches: [main]
+    paths-ignore:
+      - '**.md'
+      - 'docs/**'
+      - '.github/workflows/claude*.yml'
+
+permissions:
+  contents: read
+
+jobs:
+  backend:
+    name: Backend Tests
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up uv
+        uses: astral-sh/setup-uv@v4
+        with:
+          enable-cache: true
+
+      - name: Set up Python
+        run: uv python install
+
+      - name: Install dependencies
+        run: uv sync
+
+      - name: Run tests
+        run: uv run pytest tests/ -v
+
+  frontend:
+    name: Frontend Tests
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: frontend
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: npm
+          cache-dependency-path: frontend/package-lock.json
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Run tests
+        run: npm test