Upgrade Next.js to 16.1.6 using codemod (#80)

## Summary
Upgrades Next.js from 15.5.9 to 16.1.6 using `@next/codemod`.

## Changes

### Dependencies
- Upgraded `next` package to version 16.1.6

### Codemod Applied
The following codemods from `@next/codemod@latest` were applied:
- `remove-experimental-ppr` - No changes needed (not using
experimental_ppr)
- `remove-unstable-prefix` - No changes needed (not using unstable_
prefixes)
- `middleware-to-proxy` - Renamed `middleware.ts` to `proxy.ts` and
updated function name

### Configuration Updates

#### `next.config.ts`
- Removed deprecated `eslint` configuration (no longer supported in
Next.js 16)
- Fixed `serverActions.allowedOrigins` to safely handle undefined
`ALLOWED_ORIGIN_HOST` environment variable

#### `tsconfig.json`
- Updated `jsx` option from `"preserve"` to `"react-jsx"` (React
automatic runtime, recommended by Next.js 16)
- Added `.next/dev/types/**/*.ts` to include array

### Proxy (formerly Middleware)
- Renamed `src/middleware.ts` → `src/proxy.ts`
- Renamed `middleware` function → `proxy` function

## Build Verification
✅ `npm run build` passes successfully
✅ `npm run format:check` passes

## Notes
- ⚠️ `@aws-amplify/adapter-nextjs@1.6.0` has a peer dependency warning
(`next@">=13.5.0 <16.0.0"`), but installation completes successfully.
May need to monitor for Amplify updates supporting Next.js 16.

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---------

Co-authored-by: remote-swe-app[bot] <123456+remote-swe-app[bot]@users.noreply.github.com>

Author: remote-swe-user

webapp/next.config.ts

@@ -1,16 +1,18 @@
 import type { NextConfig } from 'next';
 
+const allowedOrigins = ['localhost:3000'];
+if (process.env.ALLOWED_ORIGIN_HOST) {
+  allowedOrigins.push(process.env.ALLOWED_ORIGIN_HOST);
+}
+
 const nextConfig: NextConfig = {
   output: 'standalone',
-  eslint: {
-    ignoreDuringBuilds: true,
-  },
   experimental: {
     webpackBuildWorker: true,
     parallelServerBuildTraces: true,
     parallelServerCompiles: true,
     serverActions: {
-      allowedOrigins: ['localhost:3000', process.env.ALLOWED_ORIGIN_HOST!],
+      allowedOrigins,
     },
   },
   typescript: {