feat: massive overhaul to manage costs (#103)

### Explanation

_No response_

### Issue

Closes #102

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### Dependencies

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

### Style

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

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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/types.rs

@@ -176,6 +176,14 @@ pub struct StateChangeEvent {
     pub conversation_id: Option<String>,
 }
 
+/// Cost information for a message
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct MessageCost {
+    pub input_tokens: u64,
+    pub output_tokens: u64,
+    pub cost_usd: f64,
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct OutputEvent {
     pub line_type: String,
@@ -183,6 +191,8 @@ pub struct OutputEvent {
     pub tool_name: Option<String>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub conversation_id: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub cost: Option<MessageCost>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -354,10 +364,31 @@ mod tests {
             content: "Test output".to_string(),
             tool_name: None,
             conversation_id: None,
+            cost: None,
         };
 
         let serialized = serde_json::to_string(&event).unwrap();
         assert!(serialized.contains("\"line_type\":\"assistant\""));
         assert!(serialized.contains("\"content\":\"Test output\""));
     }
+
+    #[test]
+    fn test_output_event_with_cost() {
+        let event = OutputEvent {
+            line_type: "assistant".to_string(),
+            content: "Test output".to_string(),
+            tool_name: None,
+            conversation_id: Some("conv-123".to_string()),
+            cost: Some(MessageCost {
+                input_tokens: 100,
+                output_tokens: 50,
+                cost_usd: 0.005,
+            }),
+        };
+
+        let serialized = serde_json::to_string(&event).unwrap();
+        assert!(serialized.contains("\"cost\":"));
+        assert!(serialized.contains("\"input_tokens\":100"));
+        assert!(serialized.contains("\"output_tokens\":50"));
+    }
 }