web: add loading screen while WASM fetches

Shows a themed spinner overlay while the large WASM bundle downloads,
improving perceived load time on slower connections.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

gui/Dioxus.toml

@@ -23,7 +23,7 @@ watch_path = ["src", "assets"]
 # CSS style file
 style = []
 # Javascript code file
-script = []
+script = ["loader.js"]
 
 [web.resource.dev]
 # serve: [dev-server] only