feat: display semantic version in sidebar footer

Reads version from package.json via Vite's define (__APP_VERSION__)
and shows it as a small label at the bottom of the sidebar.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/vite.config.js

@@ -1,8 +1,14 @@
 import { defineConfig } from 'vite'
 import vue from '@vitejs/plugin-vue'
+import { readFileSync } from 'fs'
+
+const pkg = JSON.parse(readFileSync('./package.json', 'utf-8'))
 
 export default defineConfig({
   plugins: [vue()],
+  define: {
+    __APP_VERSION__: JSON.stringify(pkg.version),
+  },
   build: {
     modulePreload: { polyfill: false },
   },