feat(frontend): enhance API credentials system and build configuration

Add real API support in demo mode with credential checking, implement build-time version injection from package.json, and refactor update checking with 24-hour caching. Migrate landing page from Vue to Astro with comprehensive UI components including Hero, Features, Benefits, and Tech Stack sections. Update CI/CD workflow with expanded cache paths and security scanner version pinned.

.github/workflows/ci-cd.yml

@@ -43,7 +43,9 @@ jobs:
       with:
         node-version: '18'
         cache: 'npm'
-        cache-dependency-path: frontend/package-lock.json
+        cache-dependency-path: |
+          frontend/package-lock.json
+          landing/package-lock.json
 
     - name: Install backend dependencies
       run: |
@@ -86,7 +88,7 @@ jobs:
       uses: actions/checkout@v4
 
     - name: Run Gosec Security Scanner
-      uses: securecodewarrior/github-action-gosec@master
+      uses: securecodewarrior/github-action-gosec@v1
       with:
         args: '-no-fail -fmt sarif -out results.sarif ./...'