Add config API and settings UI

GET/PUT /api/config for pay dates and amounts.
Settings page fetches and saves configuration.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/index.js

@@ -3,6 +3,7 @@ const express = require('express');
 const cors = require('cors');
 const path = require('path');
 const healthRouter = require('./routes/health');
+const configRouter = require('./routes/config');
 const db = require('./db');
 
 const app = express();
@@ -13,6 +14,7 @@ app.use(express.json());
 
 // API routes
 app.use('/api', healthRouter);
+app.use('/api', configRouter);
 
 // Serve static client files in production
 const clientDist = path.join(__dirname, '../../client/dist');