Add -scheme CLI flag to set the default color scheme in the output HTML

[aplgr]

Goal

Allow selecting the default color scheme via CLI when generating the HTML (e.g. -scheme default|contrast|colorblind).

Scope

• Add a new CLI flag -scheme with a small set of allowed values.
• The generated HTML should start with that scheme selected by default.
• The in-HTML selector (see issue “Add color scheme selector”) should reflect the default.

Acceptance criteria

• grovegrid ... -scheme contrast produces HTML that starts in contrast.
• Invalid value prints a helpful error and exits non-zero.
• Default behavior remains unchanged if -scheme is omitted.

Out of scope

• Custom user-defined palettes via JSON (that can be a future enhancement).