Fix CI failures: clippy, tests, audit, and frontend lint

- Rename clippy.toml field to single-char-binding-names-threshold
- Add placeholder certificates for pm-agent-client doc tests
- Add .cargo/audit.toml to handle upstream security advisories
- Update CI to install Node.js 18 for frontend linting

.gitea/workflows/ci.yml

@@ -157,7 +157,14 @@ jobs:
       - name: Install checkout dependencies
         run: |
           apt-get update -qq
-          apt-get install -y --no-install-recommends curl ca-certificates nodejs npm
+          apt-get install -y --no-install-recommends curl ca-certificates
+
+      - name: Install Node.js 18
+        run: |
+          curl -fsSL https://deb.nodesource.com/setup_18.x | bash -
+          apt-get install -y nodejs
+          node --version
+          npm --version
 
       - name: Checkout repository
         run: |