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

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

packages/platform/src/capabilities.ts

@@ -0,0 +1,14 @@
+import { platform } from "./registry";
+import type { CapabilityName } from "./types";
+
+/**
+ * Whether the current host supports a feature.
+ *
+ * A hook so components can gate on it directly, and so this can become reactive
+ * later — a browser tab gains and loses capabilities when the local bridge comes
+ * and goes. Capabilities are fixed for the life of the desktop app, so today it
+ * is a plain read.
+ */
+export function useCapability(name: CapabilityName): boolean {
+  return platform.capabilities[name];
+}