Improve WebUI generated image workflows and manual model support

[wowugeng-max]

Summary

This PR improves the WebUI image workflow and manual-model experience end to end.

WebUI generated image handling

• Render generated images returned by chat reliably, including markdown image syntax, bare local image URLs, and base64 image data.
• Handle stream terminal events so image content emitted near the end of a response is not dropped.
• Request server-side local media caching for WebUI chat image output via image_config.response_format = local_md, so generated chat images are saved in the existing local media cache and referenced as /v1/files/image?id=... instead of filling browser storage.
• Keep image and image_edit WebUI requests on local_url, matching the existing image pipeline and admin cache behavior.
• Add a download action for assistant messages that contain images.

Continuous image editing in chat/image_edit

• Let WebUI chat and image_edit carry the latest assistant image forward as context for follow-up edits.
• Do not attach previous images for normal image generation requests, so explicit new-image prompts still start a fresh image.
• Prefer manually uploaded reference images over automatic conversation context.
• Strip stale image references from older user turns when sending payloads to avoid accumulating old references.
• 

Storage and cache safety

• Avoid writing large data:image/... payloads directly into localStorage.
• Preserve lightweight image references in chat history and use server-side cached image URLs for new WebUI chat generations.
• Keep legacy/edge fallback handling for stored image references so existing sessions degrade more gracefully.

Manual model registry/admin UI

• Add a manual model registry overlay and admin model management UI.

• Route manual imagine/image-style models through the image pipeline when applicable.

• Expose manual models through the WebUI model list without requiring code changes for each entry.

Why

Before these changes, generated images could fail to render in WebUI chat, follow-up image edits did not reliably reuse the previous image, and base64 image history could quickly exhaust browser localStorage. The image model path already uses the project local media cache; this PR aligns WebUI chat image output with that same server-side cache behavior.

Test Plan

• node tests/test_webui_image_payload.mjs
• node tests/test_webui_media_normalization.mjs
• node --check app/statics/js/webui/chat.js
• uv run python -m unittest discover -s tests -p 'test_*.py'

Notes

• New WebUI chat-generated images are requested as local markdown image links and should appear in the existing admin cache under local images.
• Existing browser history entries that were previously saved as [image omitted from local storage] cannot be reconstructed because the original image bytes were already discarded by that older fallback.