Migrate from Next.js to SvelteKit

- Replace Next.js app with SvelteKit + adapter-node
- basePath /bodenfeuchte configured in svelte.config.js
- MQTT listener starts via hooks.server.ts on server boot
- Chart.js replaces Recharts for the moisture graph
- Dockerfile simplified: single build step, node build output
- Removed custom server.ts (no longer needed)

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

svelte.config.js

@@ -0,0 +1,13 @@
+import adapter from '@sveltejs/adapter-node';
+
+/** @type {import('@sveltejs/kit').Config} */
+const config = {
+	kit: {
+		adapter: adapter(),
+		paths: {
+			base: '/bodenfeuchte'
+		}
+	}
+};
+
+export default config;