The approval deny-reason prompt (and every themedPrompt dialog) used a
single-line input. Make themedPrompt always a resizable <textarea> with
chat-box keys: Enter submits (clicks the confirm button), Shift+Enter inserts
a newline. Short answers stay one keystroke; multi-line reasons (e.g. a deny
note) are now possible. No single-line variant — themedPrompt is only used by
the data-async data-prompt path, so all those dialogs get the textarea. Closes#1840.
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