feat(error): add Rust AppError enum and migrate command modules

- AppError enum with 10 variants (Database, Auth, Filesystem, Ipc, NotFound,
  Validation, Sidecar, Config, Network, Internal) + serde tag serialization
- From impls for rusqlite::Error, std::io::Error, serde_json::Error
- Migrated 9 command modules from Result<T, String> to Result<T, AppError>
- Frontend receives structured {kind, detail} objects via IPC

src-tauri/src/commands/secrets.rs

@@ -1,23 +1,24 @@
+use crate::error::AppError;
 use crate::secrets::SecretsManager;
 
 #[tauri::command]
-pub fn secrets_store(key: String, value: String) -> Result<(), String> {
-    SecretsManager::store_secret(&key, &value)
+pub fn secrets_store(key: String, value: String) -> Result<(), AppError> {
+    SecretsManager::store_secret(&key, &value).map_err(AppError::auth)
 }
 
 #[tauri::command]
-pub fn secrets_get(key: String) -> Result<Option<String>, String> {
-    SecretsManager::get_secret(&key)
+pub fn secrets_get(key: String) -> Result<Option<String>, AppError> {
+    SecretsManager::get_secret(&key).map_err(AppError::auth)
 }
 
 #[tauri::command]
-pub fn secrets_delete(key: String) -> Result<(), String> {
-    SecretsManager::delete_secret(&key)
+pub fn secrets_delete(key: String) -> Result<(), AppError> {
+    SecretsManager::delete_secret(&key).map_err(AppError::auth)
 }
 
 #[tauri::command]
-pub fn secrets_list() -> Result<Vec<String>, String> {
-    SecretsManager::list_keys()
+pub fn secrets_list() -> Result<Vec<String>, AppError> {
+    SecretsManager::list_keys().map_err(AppError::auth)
 }
 
 #[tauri::command]