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

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+// 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

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+// 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, };