feat: add session history with auto-save

- Add Rust backend for session persistence using tauri-plugin-store
- Create SessionHistoryPanel component for browsing saved sessions
- Implement debounced auto-save (2s delay) when messages are added
- Add session search, resume, and delete functionality
- Add clock icon button in StatusBar to access session history

Closes #14

src-tauri/src/sessions.rs

@@ -0,0 +1,167 @@
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+use tauri::AppHandle;
+use tauri_plugin_store::StoreExt;
+
+const SESSIONS_STORE_KEY: &str = "sessions";
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SavedSession {
+    pub id: String,
+    pub name: String,
+    pub created_at: DateTime<Utc>,
+    pub last_activity_at: DateTime<Utc>,
+    pub working_directory: String,
+    pub message_count: usize,
+    pub preview: String, // First ~100 chars of conversation for preview
+    pub messages: Vec<SavedMessage>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SavedMessage {
+    pub id: String,
+    #[serde(rename = "type")]
+    pub message_type: String,
+    pub content: String,
+    pub timestamp: DateTime<Utc>,
+    pub tool_name: Option<String>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SessionListItem {
+    pub id: String,
+    pub name: String,
+    pub created_at: DateTime<Utc>,
+    pub last_activity_at: DateTime<Utc>,
+    pub working_directory: String,
+    pub message_count: usize,
+    pub preview: String,
+}
+
+impl From<&SavedSession> for SessionListItem {
+    fn from(session: &SavedSession) -> Self {
+        SessionListItem {
+            id: session.id.clone(),
+            name: session.name.clone(),
+            created_at: session.created_at,
+            last_activity_at: session.last_activity_at,
+            working_directory: session.working_directory.clone(),
+            message_count: session.message_count,
+            preview: session.preview.clone(),
+        }
+    }
+}
+
+fn load_all_sessions(app: &AppHandle) -> Result<Vec<SavedSession>, String> {
+    let store = app
+        .store("hikari-sessions.json")
+        .map_err(|e| e.to_string())?;
+
+    match store.get(SESSIONS_STORE_KEY) {
+        Some(value) => serde_json::from_value(value.clone()).map_err(|e| e.to_string()),
+        None => Ok(Vec::new()),
+    }
+}
+
+fn save_all_sessions(app: &AppHandle, sessions: &[SavedSession]) -> Result<(), String> {
+    let store = app
+        .store("hikari-sessions.json")
+        .map_err(|e| e.to_string())?;
+
+    let value = serde_json::to_value(sessions).map_err(|e| e.to_string())?;
+    store.set(SESSIONS_STORE_KEY, value);
+    store.save().map_err(|e| e.to_string())?;
+
+    Ok(())
+}
+
+#[tauri::command]
+pub async fn list_sessions(app: AppHandle) -> Result<Vec<SessionListItem>, String> {
+    let sessions = load_all_sessions(&app)?;
+    let mut items: Vec<SessionListItem> = sessions.iter().map(SessionListItem::from).collect();
+
+    // Sort by last activity, most recent first
+    items.sort_by(|a, b| b.last_activity_at.cmp(&a.last_activity_at));
+
+    Ok(items)
+}
+
+#[tauri::command]
+pub async fn save_session(app: AppHandle, session: SavedSession) -> Result<(), String> {
+    let mut sessions = load_all_sessions(&app)?;
+
+    // Update existing or add new
+    if let Some(existing) = sessions.iter_mut().find(|s| s.id == session.id) {
+        *existing = session;
+    } else {
+        sessions.push(session);
+    }
+
+    save_all_sessions(&app, &sessions)
+}
+
+#[tauri::command]
+pub async fn load_session(app: AppHandle, session_id: String) -> Result<Option<SavedSession>, String> {
+    let sessions = load_all_sessions(&app)?;
+    Ok(sessions.into_iter().find(|s| s.id == session_id))
+}
+
+#[tauri::command]
+pub async fn delete_session(app: AppHandle, session_id: String) -> Result<(), String> {
+    let mut sessions = load_all_sessions(&app)?;
+    sessions.retain(|s| s.id != session_id);
+    save_all_sessions(&app, &sessions)
+}
+
+#[tauri::command]
+pub async fn search_sessions(app: AppHandle, query: String) -> Result<Vec<SessionListItem>, String> {
+    let sessions = load_all_sessions(&app)?;
+    let query_lower = query.to_lowercase();
+
+    let mut matching: Vec<SessionListItem> = sessions
+        .iter()
+        .filter(|s| {
+            s.name.to_lowercase().contains(&query_lower)
+                || s.preview.to_lowercase().contains(&query_lower)
+                || s.working_directory.to_lowercase().contains(&query_lower)
+                || s.messages
+                    .iter()
+                    .any(|m| m.content.to_lowercase().contains(&query_lower))
+        })
+        .map(SessionListItem::from)
+        .collect();
+
+    // Sort by last activity, most recent first
+    matching.sort_by(|a, b| b.last_activity_at.cmp(&a.last_activity_at));
+
+    Ok(matching)
+}
+
+#[tauri::command]
+pub async fn clear_all_sessions(app: AppHandle) -> Result<(), String> {
+    save_all_sessions(&app, &[])
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_session_list_item_from_saved_session() {
+        let session = SavedSession {
+            id: "test-id".to_string(),
+            name: "Test Session".to_string(),
+            created_at: Utc::now(),
+            last_activity_at: Utc::now(),
+            working_directory: "/home/test".to_string(),
+            message_count: 5,
+            preview: "Hello world".to_string(),
+            messages: vec![],
+        };
+
+        let item = SessionListItem::from(&session);
+        assert_eq!(item.id, "test-id");
+        assert_eq!(item.name, "Test Session");
+        assert_eq!(item.message_count, 5);
+    }
+}