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/components/PlatformIcons.tsx

@@ -0,0 +1,13 @@
+import { FaWindows, FaApple, FaLinux } from "react-icons/fa"
+
+export function WindowsIcon({ className = "h-3.5 w-3.5" }: { className?: string }) {
+  return <FaWindows className={className} aria-label="Windows" />
+}
+
+export function MacIcon({ className = "h-3.5 w-3.5" }: { className?: string }) {
+  return <FaApple className={className} aria-label="macOS" />
+}
+
+export function LinuxIcon({ className = "h-3.5 w-3.5" }: { className?: string }) {
+  return <FaLinux className={className} aria-label="Linux" />
+}