fix: use supported upload size limit

[MrVortexLabs]

What this PR does

Updates the image upload size limit from the unsupported 10MB literal to the NuxtHub-supported 16MB value, and keeps the widget client-side validation message in sync with the server limit.

Why

server/api/upload.post.ts used ensure.maxSize: '10MB', but the typed NuxtHub upload validator only accepts specific binary size literals such as 8MB and 16MB. This caused TypeScript to report an invalid upload size literal.

How to review

Start with server/api/upload.post.ts, then check app/components/WidgetEmbed/WidgetEmbedChat.vue to confirm the browser-side limit matches the server-side upload limit.

Checklist

• Commits are signed off (git commit -s)
• Tests pass locally (pnpm build)
• If schema changed, migration was generated (pnpm generate:migration) and committed
• If public API or env vars changed, README.md / .env.example / docs/deploy/* updated
• No secrets, internal URLs, or team member names in the diff

Local verification:

• pnpm i18n:check passes
• Upload type error for server/api/upload.post.ts is gone

Note: full typecheck still has unrelated existing errors outside this small diff.

[canghai118]

Thanks for the fix! You're right