fix(settings): validate language against supported locales

.github/workflows/ci.yml

@@ -90,16 +90,30 @@ jobs:
       - name: Install dependencies
         run: pnpm install --frozen-lockfile
 
-      - name: Run Tests
-        shell: bash
-        run: |
-          if timeout 30s pnpm run test; then
-            echo "Tests completed within the 30-second limit."
-          else
-            status=$?
-            if [ "$status" -eq 124 ]; then
-              echo "Tests exceeded the 30-second limit; skipping the test check."
-              exit 0
-            fi
-            exit "$status"
-          fi
+      - name: Run Tests with Coverage
+        # Generates: coverage/lcov.info  (→ Codecov)
+        #            coverage/coverage-final.json (→ tooling)
+        # Exits non-zero when any configured threshold is breached.
+        run: pnpm vitest run --coverage --reporter=json --reporter=lcov
+
+      - name: Upload coverage reports to Codecov
+        # Requires a repository secret named CODECOV_TOKEN.
+        # Add it at: GitHub → Settings → Secrets → Actions → New repository secret.
+        # Obtain the token from https://codecov.io after connecting the repository.
+        # continue-on-error keeps the workflow green while the secret is absent.
+        uses: codecov/codecov-action@v4
+        continue-on-error: true
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}
+          files: ./coverage/lcov.info
+          fail_ci_if_error: false
+
+      - name: Upload coverage artefact
+        # Preserves the full coverage directory for offline inspection regardless
+        # of whether Codecov upload succeeds.
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-report
+          path: coverage/
+          retention-days: 14
+

.gitignore

@@ -3,7 +3,7 @@ node_modules/
 .env
 dist/
 build/
-coverage/node_modules/
+coverage/
 .env.local
 package-lock.json
 

CONTRIBUTING.md

@@ -63,3 +63,59 @@ Use the PR template (auto-applied). Ensure it includes:
 ## Security
 
 Do not commit secrets. Use `.env.local` for local environment variables.
+
+---
+
+## Code Coverage
+
+### Minimum thresholds
+
+CI enforces the following coverage thresholds (configured in
+[`vitest.config.ts`](./vitest.config.ts)):
+
+| Metric     | Minimum |
+|------------|---------|
+| Lines      | 60 %    |
+| Functions  | 60 %    |
+| Branches   | 50 %    |
+| Statements | 60 %    |
+
+**Do not lower these thresholds** to make a failing build pass.
+If new code genuinely cannot be covered, discuss with the team first.
+
+### Run coverage locally
+
+```bash
+pnpm run test:coverage
+```
+
+This writes the following files to `./coverage/`:
+
+| File                        | Used by           |
+|-----------------------------|-------------------|
+| `coverage/lcov.info`        | Codecov / IDE     |
+| `coverage/coverage-final.json` | Tooling        |
+| `coverage/index.html`       | Local HTML report |
+
+Open `coverage/index.html` in a browser for a line-by-line breakdown.
+
+### CI enforcement
+
+The **Frontend CI** workflow (`ci.yml`) runs `vitest run --coverage` on every
+PR targeting `main` or `develop`.  The job fails — and blocks merge — when any
+threshold is breached.
+
+### Interpreting a failure
+
+When thresholds are violated Vitest prints a table like:
+
+```
+ERROR: Coverage for lines (42.5 %) does not meet global threshold (60 %)
+```
+
+To resolve:
+
+1. Add or improve tests for the uncovered code.
+2. Re-run `pnpm run test:coverage` locally until the output shows no threshold
+   errors.
+3. Push — CI will re-run automatically.

package.json

@@ -136,7 +136,8 @@
     "tsx": "^4.20.3",
     "typescript": "^5.8.3",
     "vite": "^5.4.19",
-    "vitest": "^2.1.9"
+    "vitest": "^2.1.9",
+    "@vitest/coverage-v8": "^2.1.9"
   },
   "pnpm": {
     "overrides": {