feat(agent): paste images into conversations

[DavidBabinec]

Summary

• support up to eight PNG, JPEG, or WebP images per message through paste or the image/media picker beside Send, with no conversation-wide image quota
• persist text/image and image-only turns, lazily rehydrate authenticated conversation images, and render user, assistant, and tool-result images in compact two/three-column galleries
• open images in the draggable preview and expose the same keyboard-accessible context menu on pending, chat, tool, and preview images
• copy real image bytes to the clipboard as PNG, save a MIME-correct download to the desktop, or explicitly save a separate asset to Media
• route Media saves through the canonical permissioned upload pipeline, deduplicate concurrent saves, and keep an already-open Media explorer fresh across stale list responses
• resolve selected-model image/tool capabilities authoritatively for OpenRouter and Ollama, with bounded model discovery and provider-overflow replay fallback
• harden first-send cancellation, conversation writer races, ambiguous model changes, auto-title writes, and private response caching
• document the complete attachment, replay, privacy, Media, and binary-response contracts

Why

This implements the image capability explored in #197 as a native core-agent feature, using the existing conversation, provider, Media, and tool-loop architecture instead of adding a parallel attachment or storage path.

User and developer impact

Users can attach multiple images with optional text, send image-only turns, inspect images in a compact gallery/preview, and use copy, download, or Save to Media actions. The per-message limit protects a single provider request; long conversations are not assigned an artificial image count and are bounded naturally by provider context handling.

Unsupported models and invalid or oversized images fail before persistence. Conversation images remain private, authenticated chat data unless the user explicitly chooses Save to Media, which creates a separate public Media asset and requires media.write.

The shared TypeBox request contract, browser/server limits, capability semantics, persistence behavior, replay rules, context actions, and race handling are documented in docs/features/agent.md, docs/features/media.md, and the HTTP boundary reference.

Verification

• full suite: 6,134 pass, 0 fail across 659 files
• bun run build
• bun run lint
• git diff --check
• React Doctor changed-scope review: advisory-only known/false-positive patterns; no correctness, security, compiler, or lint failure
• independent code-quality review: no remaining P0–P2 findings
• GitHub Actions: Test, Build & Typecheck, Lint, and all CodeQL analyses passed