The PromptInput onChange handler had two branches for entering bash
mode: a single-char path that just toggled the mode and a multi-char
paste path that also stripped the leading `!` from the buffer. The
single-char path returned without stripping, so typing a bare `!` into
empty input switched modes but left the literal `!` visible.
Consolidated both paths through a new pure helper `detectModeEntry`
that returns the new mode plus the stripped buffer value, so there is
no longer a branch where the mode character can leak into the buffer.
Fixes#662
