[Feature]: Clipboard paste & drag-and-drop image support in Web Chat UI

[nerdCopter]

Summary

WebUI's Agent tab, ability to paste clipbaord images or drag & drop images.

Problem statement

When using the web chat UI, there's no way to paste a screenshot from clipboard (Ctrl+V) or drag-and-drop an image file into the chat composer. Both feel like natural gestures and currently do nothing.

Proposed solution

The backend already handles [IMAGE:...] markers and multimodal payloads (fixed in #1615, #2376) — this is purely a missing frontend wiring issue.

• Bind paste event on the composer to intercept image/* clipboard items
• Bind dragover / drop events on the chat input area
• Convert the file/blob to base64 and inject as an image marker before sending

No backend changes required. Would make the web UI on par with every other channel that already supports image input (Discord, Telegram, etc.).

Non-goals / out of scope

No response

Alternatives considered

No response

Acceptance criteria

No response

Architecture impact

No response

Risk and rollback

No response

Breaking change?

No

Data hygiene checks

• I removed personal/sensitive data from examples, payloads, and logs.
• I used neutral, project-focused wording and placeholders.

[theonlyhennygod]

Closing — addressed by an existing PR.

[singlerider]

Reopening — closed in a bulk action on 2026-04-12 without verification. No PR has been identified that addresses this issue.