fix: reduce next-image payload size

[raulvbrito]

Fixes #561.

This updates the templates/next-image flow so image payloads are not passed between the browser and Next routes as base64 JSON:

• edit requests now send multipart File uploads instead of data URL JSON
• large edit attachments are resized/re-encoded in the browser before upload
• generate/edit routes return binary image responses instead of JSON-wrapped base64 data URLs
• OpenAI edits receive uploaded File objects directly on the multipart path
• legacy JSON edit requests remain supported for callers still sending data URLs

Validation:

• corepack pnpm exec tsc --noEmit
• corepack pnpm exec prettier --check ...
• git diff --check
• corepack pnpm run lint
• corepack pnpm run build

next lint and next build still report the pre-existing warnings in src/app/page.tsx and src/components/logo.tsx.

/claim #561

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