feat: enable attaching images to agent chat

[re-pixel]

Demo: https://drive.google.com/file/d/1bcmEU7CWOvt3aJRHPBG02LW93rtRkLmK/view?usp=sharing

Resolves #5136. Lets users paste or attach images (e.g. a screenshot of an error) in the canvas agent chat so the agent can act on them.

Backend

• New AgentChatImage { media_type, data } proto type; images field added to the send-message request and stored message.
• New images jsonb column on agent_session_messages (migration included).
• Images forwarded to the Anthropic provider as image content blocks alongside the text.
• gRPC handler validates media type (png/jpeg/gif/webp), base64, non-empty, ≤8 per message, and a combined ≤2.5 MB image payload per message (sized to keep the request under the gRPC limit); allows image-only messages.
• Chat history does not embed base64: message responses carry image metadata only, and bytes are served out-of-band via a new cookie-authed endpoint (GET /api/v1/agents/chats/{chatId}/messages/{messageId}/images/{index}, scoped to the session's owner). This keeps history responses small enough to stay under the gRPC/HTTP response size limit.

Frontend

• Paste images into the composer or pick them via a new attach button.
• Removable thumbnail previews before sending; transcript images load lazily from the image endpoint via <img src> (URL built from chat/message/index), rather than inline data.
• Paste only intercepts the clipboard when a supported, within-limit image is present, so unsupported/oversized pastes (and accompanying text) fall through normally.

[cursor]

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