feat(wasm): single-window web entry + trunk harness (issue #45)

The binary now compiles for wasm32 as a runnable web target:

- Split the main-window render into `view_main`, bypassing the
  multi-window id dispatch the browser cannot use.
- Add `run_web` / `boot_web`: a single-window `iced::application`
  entry for wasm (the browser canvas is the only window). Native
  keeps the multi-window `iced::daemon` via cfg-split in `main`.
- Add a `console_error_panic_hook` (wasm-only) for readable panics.
- Add `index.html` + `Trunk.toml` so `trunk serve` builds the wasm
  bundle (no-default-features → no solid3d) with COOP/COEP headers.

Native build and all 75 tests unchanged. Browser runtime (rayon
threads, canvas attach, real file I/O) is the next iteration.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/main.rs

@@ -28,5 +28,16 @@ fn main() -> iced::Result {
     if std::env::var_os("WGPU_BACKEND").is_none() {
         std::env::set_var("WGPU_BACKEND", "dx12,vulkan");
     }
-    app::run()
+
+    // Web (wasm) uses the single-window entry; native uses the multi-window
+    // daemon. Trunk calls `main` from its generated JS bootstrap.
+    #[cfg(target_arch = "wasm32")]
+    {
+        console_error_panic_hook::set_once();
+        app::run_web()
+    }
+    #[cfg(not(target_arch = "wasm32"))]
+    {
+        app::run()
+    }
 }