Allow Prompt Library to Append to Existing Input

Currently, selecting a prompt from the library automatically overwrites all existing text in the chat input box. Ideally, the default behavior should be updated to simply append the selected prompt, but if that isn't feasible, please add an 'Append to current input' checkbox within the prompt editor.

Great work on closing the previous ticket ('Prompt Library Browsing w/o Erasing Prompt') a year ago, which successfully fixed text being erased when simply opening the library! This new request focuses strictly on the replacement that happens when inserting the prompt, giving users a seamless way to combine custom text with saved templates.