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-tauri/src/config.rs

@@ -96,6 +96,22 @@ pub struct HikariConfig {
     // Custom theme colors
     #[serde(default)]
     pub custom_theme_colors: CustomThemeColors,
+
+    // Token budget settings
+    #[serde(default)]
+    pub budget_enabled: bool,
+
+    #[serde(default)]
+    pub session_token_budget: Option<u64>,
+
+    #[serde(default)]
+    pub session_cost_budget: Option<f64>,
+
+    #[serde(default = "default_budget_action")]
+    pub budget_action: BudgetAction,
+
+    #[serde(default = "default_budget_warning_threshold")]
+    pub budget_warning_threshold: f32,
 }
 
 impl Default for HikariConfig {
@@ -123,6 +139,11 @@ impl Default for HikariConfig {
             profile_avatar_path: None,
             profile_bio: None,
             custom_theme_colors: CustomThemeColors::default(),
+            budget_enabled: false,
+            session_token_budget: None,
+            session_cost_budget: None,
+            budget_action: BudgetAction::Warn,
+            budget_warning_threshold: 0.8,
         }
     }
 }
@@ -147,6 +168,22 @@ fn default_font_size() -> u32 {
     14
 }
 
+fn default_budget_action() -> BudgetAction {
+    BudgetAction::Warn
+}
+
+fn default_budget_warning_threshold() -> f32 {
+    0.8
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, Default, PartialEq)]
+#[serde(rename_all = "lowercase")]
+pub enum BudgetAction {
+    #[default]
+    Warn,
+    Block,
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize, Default, PartialEq)]
 #[serde(rename_all = "lowercase")]
 pub enum Theme {
@@ -205,6 +242,11 @@ mod tests {
         assert!(config.profile_avatar_path.is_none());
         assert!(config.profile_bio.is_none());
         assert_eq!(config.custom_theme_colors, CustomThemeColors::default());
+        assert!(!config.budget_enabled);
+        assert!(config.session_token_budget.is_none());
+        assert!(config.session_cost_budget.is_none());
+        assert_eq!(config.budget_action, BudgetAction::Warn);
+        assert!((config.budget_warning_threshold - 0.8).abs() < f32::EPSILON);
     }
 
     #[test]
@@ -232,6 +274,11 @@ mod tests {
             profile_avatar_path: None,
             profile_bio: Some("A test bio".to_string()),
             custom_theme_colors: CustomThemeColors::default(),
+            budget_enabled: true,
+            session_token_budget: Some(100000),
+            session_cost_budget: Some(1.50),
+            budget_action: BudgetAction::Block,
+            budget_warning_threshold: 0.75,
         };
 
         let json = serde_json::to_string(&config).unwrap();