Docs: backfill JSDoc, utility docs, and CLAUDE.md API/schema sections

- Add JSDoc to paychecks.js helpers: buildVirtualPaychecks, generatePaychecks, fetchPaychecksForMonth
- Add JSDoc to financing.js helpers: remainingPeriods, calcPaymentAmount, enrichPlan
- Add JSDoc to validateBillFields (bills.js) and getAllConfig (config.js)
- Add JSDoc to ThemeProvider and useTheme in ThemeContext.jsx
- Add Database Schema reference table to CLAUDE.md
- Add complete API Endpoints reference section to CLAUDE.md covering all routes

Nightshift-Task: docs-backfill
Nightshift-Ref: https://github.com/marcus/nightshift

server/src/routes/bills.js

@@ -2,6 +2,20 @@ const express = require('express');
 const router = express.Router();
 const { pool } = require('../db');
 
+/**
+ * Validate the request body for bill create/update operations.
+ *
+ * Checks that all required fields are present and within acceptable ranges.
+ * Amount is optional when `variable_amount` is true (defaults to 0 on save).
+ *
+ * @param {object} body - Request body.
+ * @param {string}        body.name              - Bill name (non-empty).
+ * @param {number|string} [body.amount]           - Bill amount; required when variable_amount is falsy.
+ * @param {number|string} body.due_day            - Day of month (1–31).
+ * @param {number|string} body.assigned_paycheck  - Which paycheck: 1 or 2.
+ * @param {boolean}       [body.variable_amount]  - Whether the bill amount varies each month.
+ * @returns {string|null} Validation error message, or null when valid.
+ */
 function validateBillFields(body) {
   const { name, amount, due_day, assigned_paycheck, variable_amount } = body;
   if (!name || name.toString().trim() === '') {