feat(leptos): frontend crate scaffold with empty app

- frontend/ crate compiles to WASM via trunk
- empty Leptos app mounted to body via wasm_bindgen(start)
- dependencies: leptos 0.7 CSR, leptos-use, lucide-leptos, gloo-net, web-sys
- workspace cargo check passes (first time the workspace compiles end-to-end)
- trunk build produces 65KB WASM + 25KB JS glue

Deviations from brief:
- lucide-leptos: 0.5 (brief) -> 3 (only version published)
- web-sys: removed invalid features ArrayBuffer/DataView (in js-sys/wasm-bindgen)
- src/main.rs -> src/lib.rs with #[wasm_bindgen(start)] to avoid bin/lib duplicate artifact

services/frontend-leptos/frontend/src/app.rs

@@ -0,0 +1,10 @@
+use leptos::prelude::*;
+
+#[component]
+pub fn App() -> impl IntoView {
+    view! {
+        <div style="display: flex; align-items: center; justify-content: center; height: 100vh; font-family: 'Poppins', sans-serif; font-size: 1.5rem; background: #0f172a; color: #e2e8f0;">
+            "Hello from Leptos"
+        </div>
+    }
+}