feat: Add About section with version info and update check (#50)

* feat: add About section with version info and update check

- Add About menu item in user dropdown
- Show frontend and backend versions separately
- Add 'Check for Updates' feature using GitHub API
- Compare versions using semver logic
- Cache update check results in sessionStorage (1 hour TTL)
- Link to GitHub repository
- Add i18n translations for EN and DE
- Extend health endpoint to return backend version

* fix: correct i18n interpolation in About modal

- Fix copyright year using dynamic interpolation
- Fix update available message (remove duplicate version placeholder)
- Add download link for available updates
- Change license to GPL-3.0

* fix: correct license to MIT

* chore: sync package.json versions to v1.3.1

frontend/vite.config.ts

@@ -1,8 +1,15 @@
 import { defineConfig } from "vite";
 import react from "@vitejs/plugin-react";
+import { readFileSync } from "fs";
+
+// Read version from package.json at build time
+const packageJson = JSON.parse(readFileSync("./package.json", "utf-8"));
 
 export default defineConfig({
   plugins: [react()],
+  define: {
+    __APP_VERSION__: JSON.stringify(packageJson.version || "unknown"),
+  },
   server: {
     port: 5173,
     strictPort: true,