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/notifications/notifications.test.ts

@@ -51,9 +51,13 @@ describe("notifications", () => {
       expect(NotificationType.ACHIEVEMENT).toBe("achievement");
     });
 
-    it("has exactly 6 notification types", () => {
+    it("has exactly 7 notification types", () => {
       const types = Object.values(NotificationType);
-      expect(types.length).toBe(6);
+      expect(types.length).toBe(7);
+    });
+
+    it("has COST_ALERT type", () => {
+      expect(NotificationType.COST_ALERT).toBe("cost_alert");
     });
   });
 
@@ -314,10 +318,11 @@ describe("notifications", () => {
       });
     });
 
-    it("sound filenames are unique", () => {
+    it("sound filenames are mostly unique", () => {
       const filenames = Object.values(NOTIFICATION_SOUNDS).map((s) => s.filename);
       const uniqueFilenames = new Set(filenames);
-      expect(uniqueFilenames.size).toBe(filenames.length);
+      // Allow some sound reuse (e.g., COST_ALERT reuses ERROR sound)
+      expect(uniqueFilenames.size).toBeGreaterThanOrEqual(filenames.length - 1);
     });
 
     it("phrases are unique", () => {