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 <noreply@anthropic.com>

src/app/view.rs

@@ -511,6 +511,13 @@ impl OpenCADStudio {
             );
         }
 
+        self.view_main()
+    }
+
+    /// The primary window: viewport, ribbon, tab bar, status bar. Split out of
+    /// `view` so the single-window web build can render it directly, bypassing
+    /// the multi-window id dispatch above (the web build has no extra windows).
+    pub fn view_main(&self) -> Element<'_, Message> {
         let i = self.active_tab;
         let tab = &self.tabs[i];
         let is_paper = tab.scene.current_layout != "Model";