Add TypeScript types for ExtensionError and improve error handling

- Add SerializedExtensionError TypeScript bindings from Rust
- Add ExtensionErrorCode enum export with ts-rs
- Create useExtensionError composable with type guards and error message extraction
- Fix developer page toast messages to show proper error messages instead of [object Object]
- Add getErrorMessage helper function for robust error handling across different error types

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/composables/useExtensionError.ts

@@ -0,0 +1,43 @@
+import type { SerializedExtensionError } from '~~/src-tauri/bindings/SerializedExtensionError'
+
+/**
+ * Type guard to check if error is a SerializedExtensionError
+ */
+export function isSerializedExtensionError(error: unknown): error is SerializedExtensionError {
+  return (
+    typeof error === 'object' &&
+    error !== null &&
+    'code' in error &&
+    'message' in error &&
+    'type' in error
+  )
+}
+
+/**
+ * Extract error message from unknown error type
+ */
+export function getErrorMessage(error: unknown): string {
+  if (isSerializedExtensionError(error)) {
+    return error.message
+  }
+
+  if (error instanceof Error) {
+    return error.message
+  }
+
+  if (typeof error === 'string') {
+    return error
+  }
+
+  return String(error)
+}
+
+/**
+ * Composable for handling extension errors
+ */
+export function useExtensionError() {
+  return {
+    isSerializedExtensionError,
+    getErrorMessage,
+  }
+}