Dynamic theme export

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New in Compose Material 3
- "Fixed" color roles are part of the M3 "ColorScheme" class since Compose Material 3 v1.4.0.

Change how "fixed" color roles are used in the app
- "Preview" screen now uses the real colors from the scheme instead of calculating them.
- "Export" screen now uses the new "ColorScheme" constructor with the "fixed" color roles included.

Small UI changes
- Remove zoom limits from "Preview"
- Add color swatches to "Export"
- Better edge-to-edge support