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/config.ts

@@ -2,6 +2,7 @@ import { writable, derived } from "svelte/store";
 import { invoke } from "@tauri-apps/api/core";
 
 export type Theme = "dark" | "light" | "high-contrast" | "custom";
+export type BudgetAction = "warn" | "block";
 
 export interface CustomThemeColors {
   bg_primary: string | null;
@@ -37,6 +38,12 @@ export interface HikariConfig {
   profile_avatar_path: string | null;
   profile_bio: string | null;
   custom_theme_colors: CustomThemeColors;
+  // Budget settings
+  budget_enabled: boolean;
+  session_token_budget: number | null;
+  session_cost_budget: number | null;
+  budget_action: BudgetAction;
+  budget_warning_threshold: number;
 }
 
 const defaultConfig: HikariConfig = {
@@ -71,6 +78,11 @@ const defaultConfig: HikariConfig = {
     text_secondary: null,
     border_color: null,
   },
+  budget_enabled: false,
+  session_token_budget: null,
+  session_cost_budget: null,
+  budget_action: "warn",
+  budget_warning_threshold: 0.8,
 };
 
 function createConfigStore() {