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/pty.rs

@@ -1,5 +1,6 @@
 use tauri::State;
 use crate::AppState;
+use crate::error::AppError;
 use crate::pty::PtyOptions;
 
 #[tauri::command]
@@ -7,13 +8,13 @@ use crate::pty::PtyOptions;
 pub fn pty_spawn(
     state: State<'_, AppState>,
     options: PtyOptions,
-) -> Result<String, String> {
-    state.pty_manager.spawn(options)
+) -> Result<String, AppError> {
+    state.pty_manager.spawn(options).map_err(AppError::sidecar)
 }
 
 #[tauri::command]
-pub fn pty_write(state: State<'_, AppState>, id: String, data: String) -> Result<(), String> {
-    state.pty_manager.write(&id, &data)
+pub fn pty_write(state: State<'_, AppState>, id: String, data: String) -> Result<(), AppError> {
+    state.pty_manager.write(&id, &data).map_err(AppError::sidecar)
 }
 
 #[tauri::command]
@@ -22,12 +23,12 @@ pub fn pty_resize(
     id: String,
     cols: u16,
     rows: u16,
-) -> Result<(), String> {
-    state.pty_manager.resize(&id, cols, rows)
+) -> Result<(), AppError> {
+    state.pty_manager.resize(&id, cols, rows).map_err(AppError::sidecar)
 }
 
 #[tauri::command]
 #[tracing::instrument(skip(state))]
-pub fn pty_kill(state: State<'_, AppState>, id: String) -> Result<(), String> {
-    state.pty_manager.kill(&id)
+pub fn pty_kill(state: State<'_, AppState>, id: String) -> Result<(), AppError> {
+    state.pty_manager.kill(&id).map_err(AppError::sidecar)
 }