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/Cargo.toml

@@ -0,0 +1,49 @@
+[package]
+name = "frontend"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+crate-type = ["cdylib", "rlib"]
+
+[dependencies]
+leptos = { version = "0.7", features = ["csr"] }
+leptos-use = "0.14"
+lucide-leptos = "3"
+shared-types = { path = "../shared-types" }
+wasm-bindgen = "0.2"
+js-sys = "0.3"
+web-sys = { version = "0.3", features = [
+    "WebSocket",
+    "MessageEvent",
+    "CloseEvent",
+    "CanvasRenderingContext2d",
+    "AudioContext",
+    "AudioBuffer",
+    "AudioBufferSourceNode",
+    "Window",
+    "Document",
+    "Element",
+    "HtmlElement",
+    "KeyboardEvent",
+    "Storage",
+    "IntersectionObserver",
+    "ResizeObserver",
+    "Url",
+    "Headers",
+    "Request",
+    "RequestInit",
+    "Response",
+    "HtmlInputElement",
+    "HtmlAudioElement",
+    "HtmlCanvasElement",
+    "MediaDevices",
+    "MediaStream",
+    "Navigator",
+    "console",
+] }
+gloo-net = "0.6"
+serde = { version = "1", features = ["derive"] }
+serde-wasm-bindgen = "0.6"
+wasm-logger = "0.2"
+console_error_panic_hook = "0.1"