Including subgroup size

[leanayu]

Description

• Addresses issue Include "n=" #17
• Added the subgroup size under each normative plot title: the sample size is dynamically computed based on the current UI filters (sex, age range, and vertebral level).

Changes
Added a helper function in callbacks.py to compute the subgroup size from the raw data.

[valosekj]

Thanks! Looks good!

Just an idea: how about showing "n" also for the "Demographics" figure?

[valosekj]

it might be nice to link the raincloudplot sex selection with the table below

UPDATE: low priority, skip for now

[leanayu]

Now, N appears when hovering the plots.
For example, in the heatmap:

Or, in the raincloud plot:

[valosekj]

Nice! Looks good!