feat(frontend): inject __APP_VERSION__ from package.json at build time

- vite.config.ts: read package.json and expose its version as a
  build-time global via Vite's `define`.
- app.d.ts: add ambient declaration so the layout's brand version
  badge (`v{__APP_VERSION__}`) type-checks.

frontend/src/app.d.ts

@@ -0,0 +1,16 @@
+// Ambient type declarations for SvelteKit + project-level build-time globals.
+
+declare global {
+	/** App version, injected from frontend/package.json at build time. */
+	const __APP_VERSION__: string;
+
+	namespace App {
+		// interface Error {}
+		// interface Locals {}
+		// interface PageData {}
+		// interface PageState {}
+		// interface Platform {}
+	}
+}
+
+export {};

frontend/vite.config.ts

@@ -1,9 +1,18 @@
 import { sveltekit } from '@sveltejs/kit/vite';
 import tailwindcss from '@tailwindcss/vite';
 import { defineConfig } from 'vite';
+import { readFileSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+
+const pkg = JSON.parse(
+	readFileSync(fileURLToPath(new URL('./package.json', import.meta.url)), 'utf8'),
+);
 
 export default defineConfig({
 	plugins: [tailwindcss(), sveltekit()],
+	define: {
+		__APP_VERSION__: JSON.stringify(pkg.version),
+	},
 	server: {
 		port: 5175,
 		proxy: {