Add playground video generation endpoints and UI; wire video parameters through frontend and backend

[LJR199887]

Motivation

• Add support for video generation from the playground with dedicated submit and fetch endpoints and pass video-specific parameters through the whole stack.
• Centralize playground token/context setup to reuse logic between chat and video flows.
• Provide a friendly UI in the playground to configure video size, seconds, and quality, and ensure video requests are handled as non-streaming tasks.

Description

• Backend: introduce PlaygroundVideoSubmit and PlaygroundVideoFetch handlers and setupPlaygroundTokenContext helper to set up token context for playground flows, and refactor Playground to use the helper before delegating to Relay via Relay(c, types.RelayFormatOpenAI).
• Router: register new routes /pg/video/generations (POST) and /pg/video/generations/:task_id (GET) and keep existing /pg/chat/completions mapping to controller.Playground.
• DTO: extend GeneralOpenAIRequest with seconds and quality fields to carry video parameters upstream.
• Frontend UI: add video parameter controls to SettingsPanel.jsx (video size, seconds, quality) and default values in DEFAULT_CONFIG.
• Frontend API/helpers: add API_ENDPOINTS.VIDEO_GENERATIONS, update buildApiPayload to include size, seconds, and quality for video models and force stream = false for them, and enrich handleApiError usage.
• Frontend request flow: extend useApiRequest to detect video-generation payloads, build simplified video request payloads (extracting prompt and image from the last user message), route requests to /pg/video/generations, treat video requests as non-streaming, handle returned task_id/video_url responses by converting them into a summary assistant message, and adjust SSE/stream dispatch logic (forceNonStream) accordingly.

Testing

• Ran backend build check with go build ./... which completed successfully.
• Ran frontend build with npm run build which completed successfully.
• Ran frontend unit checks with npm test (project test suite) and they passed.

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Codex Task