fix: coalesce fragmented paste events over SSH

Some terminals (e.g., MobaXterm) fragment large pastes into multiple
bracketed paste sequences, causing premature submit when newlines
trigger the submit handler before the full paste is received.

This adds paste coalescing with a 100ms debounce to buffer consecutive
paste events and process them as a single paste operation. Submit is
blocked while paste coalescing is active to prevent partial submissions.

fix: use correct DCS passthrough format for GNU Screen