[Feature request] Improve visualization controls - plaid-viewer

[casenave]

A few usability suggestions:

• Make it clearer that the dataset is not selected by default, for example with a dashed border or short message.
• Allow users to lock the minimum and maximum display values, which is important when comparing simulation results.
• Support discrete colour levels, for example 8 bands, to better visualize iso-contours as commonly done in FEM.
• Optionally, add box plots for global variables to show typical ranges.