refactor: convert to bun workspaces monorepo

- Move web app into apps/web/
- Create packages/shared/ with shared types
- Create plugins/decky-vault/ scaffold
- Root package.json manages workspaces only

apps/web/app/(auth)/signup/page.tsx

@@ -0,0 +1,10 @@
+import { Suspense } from "react"
+import SignupWizard from "@/components/auth/signup-wizard"
+
+export default function SignupPage() {
+    return (
+        <Suspense>
+            <SignupWizard />
+        </Suspense>
+    )
+}