[Enhancement] Render images inline in tool results and attachments with lightbox preview

Description

Tool results (Read, Bash, Markdown, Generic views) currently render only text content. Any base64 image blocks returned by a tool are dropped silently. User-uploaded image attachments in assistant messages are also displayed as filename chips with no thumbnail or preview.

This means:

• Screenshots read via the Read tool are invisible in the chat.
• Commands that emit images (charts, screenshots) produce no visible output.
• User-uploaded images can't be previewed without opening devtools.

Current behavior

• web/src/components/ToolCard/views/_results.tsx only extracts text blocks from tool result content — image blocks are skipped.
• web/src/components/AssistantChat/messages/MessageAttachments.tsx renders image attachments as plain filename chips.
• No shared image viewer component exists.

Proposed enhancement

1. Add a reusable ImageLightbox component (Portal overlay, ESC/backdrop close, open-in-new-tab).
2. Extract and render base64 image blocks when parsing tool result content in _results.tsx.
3. Wire click-to-preview into MessageAttachments for user-uploaded images.

I'll follow up with a PR. The change is additive — existing text-only tool results are unchanged.

Start with web/src/components/ToolCard/views/_results.tsx and web/src/components/AssistantChat/messages/MessageAttachments.tsx, then trace how tool result content and attachments are currently rendered. Done means a shared ImageLightbox supports portal display, ESC/backdrop close, and opening in a new tab; base64 tool images and uploaded image attachments show clickable previews while text-only results remain unchanged.