Update settings card background right away on theme switch

The section-card background (and its row dividers) were computed from a
one-time read of the current theme, so after switching theme the card
kept the old color until the screen was reopened, while the rows inside
it updated correctly. Make the two color helpers observe the theme as
state so the card recomposes on theme change like everything else.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/multiplatform/common/src/commonMain/kotlin/chat/simplex/common/ui/theme/Theme.kt

@@ -603,8 +603,9 @@ data class ThemeModeOverride (
 // cards above. DARK/BLACK: palette bg (cards already raised via founder's
 // formula in Section.kt). SIMPLEX: gradient bottom stop (darker), since the
 // canvas itself is a gradient drawn by themedBackgroundBrush.
+@Composable
 fun canvasColorForCurrentTheme(): Color {
-  val theme = CurrentColors.value
+  val theme = CurrentColors.collectAsState().value
   val c = theme.colors
   return when (theme.base) {
     DefaultTheme.LIGHT -> c.background.mixWith(c.onBackground, 0.94f)
@@ -616,8 +617,9 @@ fun canvasColorForCurrentTheme(): Color {
 // Card background color for SectionView. LIGHT: pure white (raised above the
 // off-white canvas). DARK/BLACK/SIMPLEX: founder's mixWith formula (lifts cards
 // above palette bg using onBackground tint).
+@Composable
 fun sectionCardColor(): Color {
-  val theme = CurrentColors.value
+  val theme = CurrentColors.collectAsState().value
   return if (theme.base == DefaultTheme.LIGHT) Color.White
   else theme.colors.background.mixWith(theme.colors.onBackground, 0.95f)
 }