Remove hardcoded Tailscale IP, bind server and Vite to localhost only

The Express server was binding to both 127.0.0.1 and a specific Tailscale IP
(100.84.4.113), creating two separate http.Server instances. Simplify to a
single localhost binding. Also update Vite dev server to use 127.0.0.1 instead
of the Tailscale IP and disable the HMR error overlay which obscures the UI
during development.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

vite.config.ts

@@ -12,10 +12,10 @@ export default defineConfig({
   },
   server: {
     port: 3847,
-    // Vite only supports one host. Use Tailscale IP so it's reachable
-    // from both the local machine (via the TS IP) and the tailnet.
-    // localhost:3847 won't work for the Vite dev server — use the TS IP.
-    host: "100.84.4.113",
+    host: "127.0.0.1",
+    hmr: {
+      overlay: false,
+    },
     proxy: {
       "/api": {
         target: "http://127.0.0.1:3848",