fix(tauri-app): vite bind 127.0.0.1 (not localhost)

vite's `host: host || false` config falls back to vite's
`server.host = 'localhost'` default, which on macOS resolves to
both v4 and v6 and BINDS TO v6 ONLY. The Tauri webview then
attempts to reach the dev server on v4 first (happy-eyeballs)
and gets `Connection refused`. The webview shows a blank page
(only the vite client-side scripts fail to load, with the DOM
intact) and absolutely no clicks work — not because the click
handlers are broken, but because the Svelte runtime never
loaded. The user reported 'search tut sich nix genau auch bei
compose etc.' and the previous callback-prop fix to NavRail
didn't help because the JS never ran.

Two coordinated fixes:

1. `vite.config.ts`: `host: host || '127.0.0.1'` — explicit
   IPv4-only binding that matches the `devUrl` in
   tauri.conf.json (`http://127.0.0.1:1430`).

2. `tauri.conf.json`: `devUrl: 'http://127.0.0.1:1430'`
   instead of `http://localhost:1430` — unambiguous.

The NavRail callback-prop fix (1c75d56) is kept because it
is independently correct: binding patterns on this Svelte
runtime are fragile and explicit callback props are more
robust than `bind:`.

crates/tauri-app/src-tauri/tauri.conf.json

@@ -5,7 +5,7 @@
   "identifier": "de.eifelcloud.maarcadetweet",
   "build": {
     "beforeDevCommand": "npm run dev",
-    "devUrl": "http://localhost:1430",
+    "devUrl": "http://127.0.0.1:1430",
     "beforeBuildCommand": "npm run build",
     "frontendDist": "../dist"
   },

crates/tauri-app/vite.config.ts

@@ -17,7 +17,16 @@ export default defineConfig(async () => ({
   server: {
     port: 1430,
     strictPort: true,
-    host: host || false,
+    // Always bind to IPv4 127.0.0.1 explicitly. `host || false` lets
+    // vite pick "localhost" which on macOS resolves to both v4 and v6
+    // and binds to v6 by default — the Tauri webview then tries v4
+    // first and gets a connection refused. The explicit v4 address
+    // matches `tauri.conf.json`'s `devUrl` so the webview connects
+    // reliably. HMR via Tauri (cross-platform) still works because
+    // vite binds both v4 and v6 in modern versions when the address
+    // is the v4 loopback; if that breaks for someone we can use
+    // `0.0.0.0` to listen on all interfaces instead.
+    host: host || "127.0.0.1",
     hmr: host
       ? { protocol: "ws", host, port: 1431 }
       : undefined,