feat: massive overhaul to manage costs (#103)

### Explanation

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### Issue

Closes #102

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### Dependencies

- [ ] I have pinned the dependencies to a specific patch version.

### Style

- [ ] I have run the linter and resolved any errors.
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### Tests

- [ ] My contribution adds new code, and I have added tests to cover it.
- [ ] My contribution modifies existing code, and I have updated the tests to reflect these changes.
- [ ] All new and existing tests pass locally with my changes.
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### Documentation

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Reviewed-on: #103
Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Co-committed-by: Naomi Carrigan <commits@nhcarrigan.com>

src-tauri/src/commands.rs

@@ -556,6 +556,97 @@ pub async fn rename_path(old_path: String, new_path: String) -> Result<(), Strin
     Ok(())
 }
 
+// ==================== Cost Tracking Commands ====================
+
+const COST_HISTORY_STORE_KEY: &str = "cost_history";
+
+#[tauri::command]
+pub async fn get_cost_summary(app: AppHandle, days: u32) -> Result<crate::cost_tracking::CostSummary, String> {
+    let history = load_cost_history(&app).await;
+    Ok(history.get_summary(days))
+}
+
+#[tauri::command]
+pub async fn get_cost_alerts(app: AppHandle) -> Result<Vec<crate::cost_tracking::CostAlert>, String> {
+    let mut history = load_cost_history(&app).await;
+    let alerts = history.check_alerts();
+
+    // Save updated alert state
+    save_cost_history(&app, &history).await?;
+
+    Ok(alerts)
+}
+
+#[tauri::command]
+pub async fn set_cost_alert_thresholds(
+    app: AppHandle,
+    daily: Option<f64>,
+    weekly: Option<f64>,
+    monthly: Option<f64>,
+) -> Result<(), String> {
+    let mut history = load_cost_history(&app).await;
+    history.set_alert_thresholds(daily, weekly, monthly);
+    save_cost_history(&app, &history).await
+}
+
+#[tauri::command]
+pub async fn export_cost_csv(app: AppHandle, days: u32) -> Result<String, String> {
+    let history = load_cost_history(&app).await;
+    Ok(history.export_csv(days))
+}
+
+#[tauri::command]
+pub async fn get_today_cost(app: AppHandle) -> Result<f64, String> {
+    let history = load_cost_history(&app).await;
+    Ok(history.get_today_cost())
+}
+
+#[tauri::command]
+pub async fn get_week_cost(app: AppHandle) -> Result<f64, String> {
+    let history = load_cost_history(&app).await;
+    Ok(history.get_week_cost())
+}
+
+#[tauri::command]
+pub async fn get_month_cost(app: AppHandle) -> Result<f64, String> {
+    let history = load_cost_history(&app).await;
+    Ok(history.get_month_cost())
+}
+
+/// Add cost to history (called internally when stats are updated)
+pub async fn record_cost(app: &AppHandle, input_tokens: u64, output_tokens: u64, cost_usd: f64) {
+    let mut history = load_cost_history(app).await;
+    history.add_cost(input_tokens, output_tokens, cost_usd);
+    let _ = save_cost_history(app, &history).await;
+}
+
+/// Record a new session
+pub async fn record_session(app: &AppHandle) {
+    let mut history = load_cost_history(app).await;
+    history.increment_sessions();
+    let _ = save_cost_history(app, &history).await;
+}
+
+async fn load_cost_history(app: &AppHandle) -> crate::cost_tracking::CostHistory {
+    let store = match app.store("hikari-cost-history.json") {
+        Ok(s) => s,
+        Err(_) => return crate::cost_tracking::CostHistory::new(),
+    };
+
+    match store.get(COST_HISTORY_STORE_KEY) {
+        Some(value) => serde_json::from_value(value.clone()).unwrap_or_default(),
+        None => crate::cost_tracking::CostHistory::new(),
+    }
+}
+
+async fn save_cost_history(app: &AppHandle, history: &crate::cost_tracking::CostHistory) -> Result<(), String> {
+    let store = app.store("hikari-cost-history.json").map_err(|e| e.to_string())?;
+    let value = serde_json::to_value(history).map_err(|e| e.to_string())?;
+    store.set(COST_HISTORY_STORE_KEY, value);
+    store.save().map_err(|e| e.to_string())?;
+    Ok(())
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;