webui : fix location of pasted input within the user message #22962

In the WebUI, I believe the current logic for pasting large text in the input is like this:

# constructed prompt in this case
Summarize in 1 sentence
[pasted text]

Instead, it's more practical to have the order in reverse:

# constructed prompt in this case
[pasted text]
Summarize in 1 sentence

This allows more efficient prefix reuse when sending different queries with the same input data.