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/persistence.rs

@@ -1,5 +1,6 @@
 use tauri::State;
 use crate::AppState;
+use crate::error::AppError;
 use crate::session::{AgentMessageRecord, ProjectAgentState, SessionMetric, SessionAnchorRecord};
 
 // --- Agent message persistence ---
@@ -11,7 +12,7 @@ pub fn agent_messages_save(
     project_id: String,
     sdk_session_id: Option<String>,
     messages: Vec<AgentMessageRecord>,
-) -> Result<(), String> {
+) -> Result<(), AppError> {
     state.session_db.save_agent_messages(
         &session_id,
         &project_id,
@@ -24,7 +25,7 @@ pub fn agent_messages_save(
 pub fn agent_messages_load(
     state: State<'_, AppState>,
     project_id: String,
-) -> Result<Vec<AgentMessageRecord>, String> {
+) -> Result<Vec<AgentMessageRecord>, AppError> {
     state.session_db.load_agent_messages(&project_id)
 }
 
@@ -34,7 +35,7 @@ pub fn agent_messages_load(
 pub fn project_agent_state_save(
     state: State<'_, AppState>,
     agent_state: ProjectAgentState,
-) -> Result<(), String> {
+) -> Result<(), AppError> {
     state.session_db.save_project_agent_state(&agent_state)
 }
 
@@ -42,7 +43,7 @@ pub fn project_agent_state_save(
 pub fn project_agent_state_load(
     state: State<'_, AppState>,
     project_id: String,
-) -> Result<Option<ProjectAgentState>, String> {
+) -> Result<Option<ProjectAgentState>, AppError> {
     state.session_db.load_project_agent_state(&project_id)
 }
 
@@ -52,7 +53,7 @@ pub fn project_agent_state_load(
 pub fn session_metric_save(
     state: State<'_, AppState>,
     metric: SessionMetric,
-) -> Result<(), String> {
+) -> Result<(), AppError> {
     state.session_db.save_session_metric(&metric)
 }
 
@@ -61,7 +62,7 @@ pub fn session_metrics_load(
     state: State<'_, AppState>,
     project_id: String,
     limit: i64,
-) -> Result<Vec<SessionMetric>, String> {
+) -> Result<Vec<SessionMetric>, AppError> {
     state.session_db.load_session_metrics(&project_id, limit)
 }
 
@@ -71,7 +72,7 @@ pub fn session_metrics_load(
 pub fn session_anchors_save(
     state: State<'_, AppState>,
     anchors: Vec<SessionAnchorRecord>,
-) -> Result<(), String> {
+) -> Result<(), AppError> {
     state.session_db.save_session_anchors(&anchors)
 }
 
@@ -79,7 +80,7 @@ pub fn session_anchors_save(
 pub fn session_anchors_load(
     state: State<'_, AppState>,
     project_id: String,
-) -> Result<Vec<SessionAnchorRecord>, String> {
+) -> Result<Vec<SessionAnchorRecord>, AppError> {
     state.session_db.load_session_anchors(&project_id)
 }
 
@@ -87,7 +88,7 @@ pub fn session_anchors_load(
 pub fn session_anchor_delete(
     state: State<'_, AppState>,
     id: String,
-) -> Result<(), String> {
+) -> Result<(), AppError> {
     state.session_db.delete_session_anchor(&id)
 }
 
@@ -95,7 +96,7 @@ pub fn session_anchor_delete(
 pub fn session_anchors_clear(
     state: State<'_, AppState>,
     project_id: String,
-) -> Result<(), String> {
+) -> Result<(), AppError> {
     state.session_db.delete_project_anchors(&project_id)
 }
 
@@ -104,6 +105,6 @@ pub fn session_anchor_update_type(
     state: State<'_, AppState>,
     id: String,
     anchor_type: String,
-) -> Result<(), String> {
+) -> Result<(), AppError> {
     state.session_db.update_anchor_type(&id, &anchor_type)
 }