feat: massive overhaul to manage costs (#103)

### Explanation

_No response_

### Issue

Closes #102

### Attestations

- [ ] I have read and agree to the [Code of Conduct](https://docs.nhcarrigan.com/community/coc/)
- [ ] I have read and agree to the [Community Guidelines](https://docs.nhcarrigan.com/community/guide/).
- [ ] My contribution complies with the [Contributor Covenant](https://docs.nhcarrigan.com/dev/covenant/).

### Dependencies

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

### Style

- [ ] I have run the linter and resolved any errors.
- [ ] My pull request uses an appropriate title, matching the conventional commit standards.
- [ ] My scope of feat/fix/chore/etc. correctly matches the nature of changes in my pull request.

### 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.
- [ ] Code coverage remains at or above the configured threshold.

### Documentation

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### Versioning

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

src/lib/stores/costTracking.ts

@@ -0,0 +1,182 @@
+import { writable, derived } from "svelte/store";
+import { invoke } from "@tauri-apps/api/core";
+import { notificationManager } from "$lib/notifications/notificationManager";
+
+// Types matching Rust backend
+export interface DailyCost {
+  date: string;
+  input_tokens: number;
+  output_tokens: number;
+  cost_usd: number;
+  messages_sent: number;
+  sessions_count: number;
+}
+
+export interface CostSummary {
+  period_days: number;
+  total_input_tokens: number;
+  total_output_tokens: number;
+  total_cost: number;
+  total_messages: number;
+  total_sessions: number;
+  average_daily_cost: number;
+  daily_breakdown: DailyCost[];
+}
+
+export type AlertType = "Daily" | "Weekly" | "Monthly";
+
+export interface CostAlert {
+  alert_type: AlertType;
+  threshold: number;
+  current_cost: number;
+}
+
+export interface CostAlertThresholds {
+  daily: number | null;
+  weekly: number | null;
+  monthly: number | null;
+}
+
+// Store state
+interface CostTrackingState {
+  todayCost: number;
+  weekCost: number;
+  monthCost: number;
+  summary: CostSummary | null;
+  alerts: CostAlert[];
+  thresholds: CostAlertThresholds;
+  isLoading: boolean;
+  lastUpdated: Date | null;
+}
+
+const defaultState: CostTrackingState = {
+  todayCost: 0,
+  weekCost: 0,
+  monthCost: 0,
+  summary: null,
+  alerts: [],
+  thresholds: { daily: null, weekly: null, monthly: null },
+  isLoading: false,
+  lastUpdated: null,
+};
+
+function createCostTrackingStore() {
+  const { subscribe, set, update } = writable<CostTrackingState>(defaultState);
+
+  return {
+    subscribe,
+
+    async refresh() {
+      update((s) => ({ ...s, isLoading: true }));
+
+      try {
+        const [todayCost, weekCost, monthCost, alerts] = await Promise.all([
+          invoke<number>("get_today_cost"),
+          invoke<number>("get_week_cost"),
+          invoke<number>("get_month_cost"),
+          invoke<CostAlert[]>("get_cost_alerts"),
+        ]);
+
+        update((s) => ({
+          ...s,
+          todayCost,
+          weekCost,
+          monthCost,
+          alerts,
+          isLoading: false,
+          lastUpdated: new Date(),
+        }));
+
+        // Trigger notifications for any new alerts
+        if (alerts.length > 0) {
+          for (const alert of alerts) {
+            const message = getAlertMessage(alert);
+            notificationManager.notifyCostAlert(message);
+          }
+        }
+
+        return alerts;
+      } catch (error) {
+        console.error("Failed to refresh cost tracking:", error);
+        update((s) => ({ ...s, isLoading: false }));
+        return [];
+      }
+    },
+
+    async getSummary(days: number): Promise<CostSummary | null> {
+      try {
+        const summary = await invoke<CostSummary>("get_cost_summary", { days });
+        update((s) => ({ ...s, summary }));
+        return summary;
+      } catch (error) {
+        console.error("Failed to get cost summary:", error);
+        return null;
+      }
+    },
+
+    async setAlertThresholds(thresholds: CostAlertThresholds) {
+      try {
+        await invoke("set_cost_alert_thresholds", {
+          daily: thresholds.daily,
+          weekly: thresholds.weekly,
+          monthly: thresholds.monthly,
+        });
+        update((s) => ({ ...s, thresholds }));
+      } catch (error) {
+        console.error("Failed to set alert thresholds:", error);
+      }
+    },
+
+    async exportCsv(days: number): Promise<string | null> {
+      try {
+        return await invoke<string>("export_cost_csv", { days });
+      } catch (error) {
+        console.error("Failed to export CSV:", error);
+        return null;
+      }
+    },
+
+    reset() {
+      set(defaultState);
+    },
+  };
+}
+
+export const costTrackingStore = createCostTrackingStore();
+
+// Derived stores for formatted values
+export const formattedCosts = derived(costTrackingStore, ($store) => ({
+  today: formatCost($store.todayCost),
+  week: formatCost($store.weekCost),
+  month: formatCost($store.monthCost),
+  todayRaw: $store.todayCost,
+  weekRaw: $store.weekCost,
+  monthRaw: $store.monthCost,
+}));
+
+// Helper functions
+export function formatCost(cost: number): string {
+  if (cost < 0.01) {
+    return `$${cost.toFixed(4)}`;
+  }
+  if (cost < 1) {
+    return `$${cost.toFixed(3)}`;
+  }
+  return `$${cost.toFixed(2)}`;
+}
+
+export function formatAlertType(type: AlertType): string {
+  switch (type) {
+    case "Daily":
+      return "Today";
+    case "Weekly":
+      return "This Week";
+    case "Monthly":
+      return "This Month";
+  }
+}
+
+export function getAlertMessage(alert: CostAlert): string {
+  const period = formatAlertType(alert.alert_type);
+  return `${period}'s spending (${formatCost(alert.current_cost)}) has exceeded your ${formatCost(alert.threshold)} threshold`;
+}