feat(chat): render images in agent turns, paste-to-send from the input

[brettchien]

Summary

• Closes Chat panel: render images in agent turns + paste-to-send images from the input #158. The ACP wire already supports image ContentBlocks (crates/openab-gateway/src/adapters/acp_schema.rs:5311 in openabdev/openab) — the gap was the bridge and the console, not the protocol.
• Bridge (crates/acp-tunnel, src-tauri/src/remote.rs): parse_inbound now classifies session/update content by its type (text → Inbound::AgentChunk, image → new Inbound::AgentImage), forwarded to the panel as its own agent-update (kind: "image"). Session::prompt takes a new images: &[PromptImage] param and appends image blocks after the text block; agent_prompt/OutMsg::Prompt thread it through.
• Console render (console/src/chat.ts): ChatTurn gains an optional images field; appendImage mirrors appendChunk's open-turn rule. Images render as data: URI <img>s behind a mime allow-list (png/jpeg/gif/webp) + a base64-shape check, since agent-sourced content is less trusted than markdown text (same concern the issue flagged for a DOMPurify-safety pass).
• Console input (console/src/chatPanel.ts): a paste handler on the textarea picks up clipboard image items, base64-encodes them client-side, and shows a removable preview strip above the input; submit() bundles pending images into the next agent_prompt call and allows an image-only send (empty text).
• Size decision (issue's open question): 5MB raw per image, 4 images per message, enforced on paste; render side caps at ~8MB decoded/mimeType allow-list. No downscale in this slice — an oversized paste is rejected with a note rather than silently shrunk; that's a reasonable follow-up if it turns out to matter in practice.

Test plan

• cargo test -p acp-tunnel — 38/38 pass (12 new: prompt image-block building, session/update image classification)
• cargo check --workspace — clean (workspace excludes src-tauri, which needs GTK/webkit system libs this Linux container doesn't have — same reason desktop.yml's Linux compile-validation stage is disabled; src-tauri's edits are small/mechanical and covered by the macOS desktop.yml job on this PR)
• npm run typecheck (console) — clean
• npm test (console) — 117/117 pass (10 new: appendImage, isRenderableImage, turnHtml image rendering)
• Manual verification against a live /acp gateway (paste an image in, confirm an agent-sent image renders) — not done in this session, no live gateway available here

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