feat: add scopeClassName to ThemeProvider and merge context values

[noahwaldner]

Added scopeClassName prop

This prop can be used to set a classname for scoped styles, such as guideline blocks.
The value is passed to all portals through the theme context, so also apply in dialogs etc.

I avoided using the name className as it is misleading in what the prop does. It does not set the classname of the Provider component, but add one additional class to the existing set.

Context merging

Previously, every nested ThemeProvider overrode the theme context with the default values.
We now merge the context and only override the values explicitly passed to the nested ThemeProvider

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[github-actions]

Lead time: 3 hours, 20 minutes, 59 seconds (3.35 total hours) from first commit to close.
Review time: 3 hours, 12 minutes, 30 seconds (3.21 total hours) from ready for review to close.

• First commit: 12.5.2026, 11:56:15.
• Ready for review: 12.5.2026, 12:04:44.
• Closed: 12.5.2026, 15:17:14.