Make dioxus_native::launch_cfg work on WASM

#434 in DioxusLabs/blitz — merged 2026-05-12

Adds Config::with_font_ctx and seeds a with_append(true) wasm canvas default so dioxus_native::launch_cfg compiles and runs on wasm32 (with the tokio-backed blitz_net::Provider gated off there). The seven_guis and todomvc examples collapse their hand-rolled wasm entry points to a single launch_cfg call.