[klausgibin]

Summary

• Add Higgsfield as a manually selected second backend for the existing Seedance 2.0 video model, preserving Kie.ai as the default.
• Save Higgsfield Key ID and Secret in the app's local SQLite settings and keep SDK calls server-side.
• Use the official @higgsfield/client/v2 SDK for Seedance 2.0 text-to-video, including asynchronous polling, result storage and error handling.
• Show provider selection and missing-key status in Settings and Video Generator.

Validation

• corepack pnpm exec tsc --noEmit — passed
• corepack pnpm build — passed
• corepack pnpm test:higgsfield — passed (mock API: credentials, payload, queued/in-progress/completed, failure, 429, Kie route selection)
• Browser check — Settings and Video Generator show provider selection; switching Kie ↔ Higgsfield works without restart.
• corepack pnpm lint — repository currently reports 66 pre-existing errors. The touched files have the same lint error rules on the original main branch.

Scope and limits

• Higgsfield supports only Seedance 2.0 prompt-only text-to-video in this PR. Media references still use Kie.
• No real paid Higgsfield or Kie generation was attempted without credentials.
• The SDK's TypeScript v2 subscribe polls automatically, but this MVP does not resume an in-flight Higgsfield job after local server restart.
• No webhook, automatic fallback or automatic resubmission is included.

See docs/HIGGSFIELD.md for setup and next-PR plan.