fix(ui): default LocalContentColor to onSurface so translucent surfac…

…es keep theme-aware content (xingguangcuican6666#228)

Material3 1.5.0-alpha19's MaterialTheme no longer provides
LocalContentColor, leaving it at the Color.Black default. Card
surfaces made translucent by the blur / custom-background feature
never match a scheme role in contentColorFor(), so it falls back to
LocalContentColor - black text/icons on dark surfaces (invisible in
dark mode, fine in light). Provide the standard onSurface default at
the AbkTheme root, matching stable Material3 behavior.