Add a typed platform package to decouple the frontend from Tauri (#539)

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

packages/platform/src/index.ts

@@ -0,0 +1,15 @@
+import { setPlatform } from "./registry";
+import { createTauriPlatform } from "./tauri";
+
+// Desktop is the only host today, so it is installed unconditionally and
+// synchronously — several modules call commands while the module graph is still
+// evaluating, so there is no later moment to do this in.
+//
+// This line is the swap point. A browser build selects its own host here, and
+// because nothing else in the app imports a host directly, that is the whole
+// change.
+setPlatform(createTauriPlatform());
+
+export * from "./capabilities";
+export { platform, setPlatform } from "./registry";
+export * from "./types";