Sync Mark-10 gauge's live unit into ChannelConfig instead of a fixed default

Mark10Layer._parse() already tracked whichever unit suffix the gauge
last reported (lb/kgF/N/ozF — the gauge's physical unit button cycles
these independently of this app). serial_device.py hardcoded the
"force" channel's unit to "N" and never updated it, so switching units
on the gauge itself was invisible here.

Adds Mark10Layer.current_unit and has SerialDevice.read_channels()
keep the "force" ChannelConfig's unit in sync with it on every poll.
This fixes the unit shown in anything that reads ChannelConfig.unit
live — new plots, channel pickers, CSV log headers — but does not
relabel the Y-axis of an already-open plot pane, since plot axis
labels are baked into PlotConfig.y_label once at plot-build time, not
re-read from the channel live. Making an open pane's axis relabel
itself would need new signal plumbing from the device through
AcquisitionEngine to the strip chart — a bigger, separate change.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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