feat: show server version in sidebar header

Version injected at build time via VITE_APP_VERSION env var.
CI sets it to branch@sha. Falls back to 'dev' in local dev.
Displayed next to "Cameleer" in the sidebar header.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/vite.config.ts

@@ -6,6 +6,9 @@ import react from '@vitejs/plugin-react';
 const apiTarget = process.env.VITE_API_TARGET || 'http://localhost:8081';
 
 export default defineConfig({
+  define: {
+    __APP_VERSION__: JSON.stringify(process.env.VITE_APP_VERSION || 'dev'),
+  },
   plugins: [react()],
   server: {
     proxy: {