Theme: document why canvasColorForCurrentTheme reads CurrentColors.value directly

Reviewer asked why this helper uses CurrentColors.value.base instead of the
Compose MaterialTheme/CompositionLocal route. Reason is that the helper is
intentionally callable from both @Composable bodies and DrawScope (inside
sectionItemDivider's drawWithContent), and DrawScope can't invoke @Composable
getters. Add a paragraph to the doc-comment so future readers don't try to
'fix' it back to MaterialTheme.colors and break the divider draw path.

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

@@ -603,6 +603,13 @@ 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.
+//
+// Reads CurrentColors.value directly rather than via MaterialTheme/CompositionLocal
+// because this helper is called from two scopes: @Composable bodies (SectionView,
+// SectionDivider) and DrawScope (sectionItemDivider's drawWithContent). DrawScope
+// can't call @Composable getters, so a snapshot read keeps one helper usable in
+// both places. State invalidation still works — CurrentColors is a MutableStateFlow,
+// reads are tracked by Compose snapshot system.
 fun canvasColorForCurrentTheme(): Color {
   val theme = CurrentColors.value
   val c = theme.colors