Webui: copy prompt and attachments

[ServeurpersoCom]

Make sure to read the contributing guidelines before submitting a PR

Description

This PR improves the copy functionality in the WebUI to handle messages with attachments (pasted content).

Changes

First commit: Enable copying text from pasted attachments when message content is empty

Second commit: Copy both prompt and all attachments separated by blank lines

Behavior

• If a message has no prompt text but contains a pasted attachment, copying now retrieves the attachment content
• If a message has both prompt text and attachments, copying concatenates them with newline separators in order: prompt first, then all attachments

Use case

Allows copying long prompts (converted to "pasted" attachments) from one chat to another, and properly handles messages with multiple attachments.

Video:

Copy-Pasted-Only-Prompt.mp4

Expected result:

Prompt\n
\n
Attach1\n
\n
Attach2\n
\n
...

Closes #17834

[ServeurpersoCom]

I'm going to make a video