Matches #5910's text-only footprint. Removes image-vector embedding
(encode_images, _stream_image_chunks, the _BGEVLAdapter CLIP shim), the
multimodal `mode`/KBMode concept + VL embedders (single text embedder
now), the mode selector UI across the KB dialogs + thread settings, the
MM badges, the /images serving route, and the dead image rendering in
the search tool card. Captioning (figure text spliced into markdown)
stays — #5910 keeps it too. DB mode/image columns left dormant (no
migration). RagDefaultsSection dropped (no controls left).
2366104c4f