v0.6.31

Show a dim baseline for an expanded sparkline with no data

When a signal is expanded before any reading arrives (e.g. no source
connected), the sparkline window was all gaps, so it rendered as a blank line
and looked like nothing happened ("sparkline doesn't appear"). Now an expanded
signal with no data shows a dim dashed baseline spanning the sparkline track,
so the empty graph is visibly present and clearly waiting for data. Once
readings arrive it switches to the live sparkline as before.

392 tests green; ruff + mypy clean.