Normalize callback prop naming to on[Noun][Verb] convention

Rename onAmountSave → onPaycheckAmountSave and onGenerate →
onPaycheckGenerate in PaycheckColumn to match the on[Noun][Verb]
pattern used by all other callback props (onBillPaidToggle,
onBillAmountSave, etc.). Document the convention in CLAUDE.md.

Nightshift-Task: event-taxonomy
Nightshift-Ref: https://github.com/marcus/nightshift

CLAUDE.md

@@ -77,6 +77,8 @@ cd client && npm run test:watch
 - Export pure functions (validators, formatters, etc.) for direct testing
 - Run `npm test` in both `server/` and `client/` before committing
 
+**Callback prop naming convention:** React callback props follow `on[Noun][Verb]` (e.g., `onBillPaidToggle`, `onPaycheckAmountSave`, `onPaycheckGenerate`). Event handler functions in the parent component use the `handle[Action]` prefix (e.g., `handleAmountSave`, `handleBillPaidToggle`).
+
 ## Application Structure
 
 The default route `/` renders the paycheck-centric main view (`client/src/pages/PaycheckView.jsx`). It shows the current month's two paychecks side-by-side with bills, paid status, one-time expenses, and remaining balance. Month navigation (prev/next) fetches data via `GET /api/paychecks?year=&month=`.