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"
   },