Introduce scrimMedium and scrimStrong color tokens to the shared
theme color tokens and propagate them through constructors, copyWith,
and lerp so scrim values interpolate and can be overridden.
Define scrim tokens as black with different alpha values per theme
mode (lighter alpha in light mode stronger in dark mode) to create
a consistent darkening effect for overlays.
Refactor ChatPage to use the new scrim tokens for drawer scrims and
format a RegExp call for readability. This replaces previous use of
overlay tokens for platform-specific scrims to provide clearer
semantics and better visual control for modal/drawer backdrops.
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