dashboard: Add deviation chart with baseline and relative comparision

Introduces a new function `render_deviation_comparison_chart` to
visualize relative performance changes across configs or SUTs.

Users can select a baseline config or SUT via dropdown to compare
against others. The chart shows deviation bars, color-coded by
change type: green for improvements, red for regressions, and grey
for neutral changes within a configurable threshold (default 0.1%).

The logic aligns with `compute_change` from show.py and supports
relative/absolute views and filtering by change type via sidebar.

Signed-off-by: Aishwarya TCV <aishwarya.tcv@arm.com>