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-tauri/src/extension/error.rs

@@ -1,10 +1,12 @@
 // src-tauri/src/extension/error.rs
 use thiserror::Error;
+use ts_rs::TS;
 
 use crate::database::error::DatabaseError;
 
 /// Error codes for frontend handling
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, TS)]
+#[ts(export)]
 pub enum ExtensionErrorCode {
     SecurityViolation = 1000,
     NotFound = 1001,
@@ -25,6 +27,17 @@ pub enum ExtensionErrorCode {
     Installation = 5000,
 }
 
+/// Serialized representation of ExtensionError for TypeScript
+#[derive(Debug, Clone, serde::Serialize, TS)]
+#[ts(export)]
+pub struct SerializedExtensionError {
+    pub code: u16,
+    #[serde(rename = "type")]
+    pub error_type: String,
+    pub message: String,
+    pub extension_id: Option<String>,
+}
+
 impl serde::Serialize for ExtensionErrorCode {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where