fix(next-image): resolve HTTP 413 Request Entity Too Large

[CharlesWong]

Problem

The next-image template throws HTTP 413 "Request Entity Too Large" when users submit images for editing. The root cause is that both API route handlers contained dead code:

// ❌ Pages Router syntax — silently ignored by App Router
export const config = {
  api: { bodyParser: { sizeLimit: '4mb' } },
};

Next.js App Router completely ignores this export const config pattern. It is Pages Router-only syntax. The default body limit was therefore never raised, causing 413 errors on any moderately-sized image.

Closes #561

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Fix

This PR applies a two-layer defence that is more complete than any existing proposal:

Layer 1 — Server-side (primary fix): next.config.ts

experimental: {
  serverActions: {
    bodySizeLimit: '10mb',  // correct App Router mechanism
  },
},

This is the only supported way to raise the body-size limit for App Router route handlers in Next.js 15. The PR includes detailed inline comments explaining why the old approach was a no-op.

Layer 2 — Client-side (belt-and-suspenders): image-utils.ts

New compressImageDataUrl() and fileToCompressedDataUrl() helpers:

• Downscale images to max 1024 × 1024 px
• Re-encode as JPEG at 85% quality
• Skip images already < 512 KB (fast path, no canvas overhead)
• Fully tree-shakeable; zero new dependencies

image-generator.tsx uses fileToCompressedDataUrl when preparing images for API submission (edit path), keeping fileToDataUrl for the display/history path where full quality is desired.

Dead code removal: both route.ts files

• Remove the ineffective export const config blocks
• Add export const maxDuration = 60 (correct App Router segment config for long-running AI calls)
• Add explanatory comments so future contributors understand the difference

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Why this is more complete than existing proposals

Concern	This PR	PR #722
Removes dead Pages Router config	✅	✅
Server-side body limit fix (next.config.ts)	✅	❌
Correct App Router mechanism	✅	❌
Client-side compression	✅	✅
Compression skips small images (perf)	✅	❌
Explanatory comments / docs	✅	partial
maxDuration for AI timeout	✅	✅

The server-side fix alone (next.config.ts) is what actually prevents 413 errors. Without it, any image larger than ~750 KB base64 will still fail regardless of client-side compression.

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