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/bindings/ExtensionErrorCode.ts

@@ -0,0 +1,6 @@
+// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
+
+/**
+ * Error codes for frontend handling
+ */
+export type ExtensionErrorCode = "SecurityViolation" | "NotFound" | "PermissionDenied" | "MutexPoisoned" | "Database" | "Filesystem" | "FilesystemWithPath" | "Http" | "Shell" | "Manifest" | "Validation" | "InvalidPublicKey" | "InvalidSignature" | "InvalidActionString" | "SignatureVerificationFailed" | "CalculateHash" | "Installation";

src-tauri/bindings/SerializedExtensionError.ts

@@ -0,0 +1,6 @@
+// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
+
+/**
+ * Serialized representation of ExtensionError for TypeScript
+ */
+export type SerializedExtensionError = { code: number, type: string, message: string, extension_id: string | null, };

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