fix(herdi-win): cut the prompt preview from the end the relay does

An approval's question, and the options parsed back out of it, are the last thing on
the pane. The relay keeps content[-500:] in blocked_message; this kept the first 500
characters instead, so a long prompt reached the toast as the scrollback above the
question, with no buttons on it because DetectOptions had nothing left to find.

The test that exists to catch exactly this asserted the relay's numbers against the
relay's source and got them wrong -- it wanted 50/20/500 where read_pane() has always
fetched 100 lines and kept the last 50 -- so it had never passed. It now asserts what
each side really does, and that both cut off the same end.