fix: resolve TypeScript errors caught by typecheck

- _layout: replace invalid SFSymbols7_0 name "calendar.fill" with
  "calendar.circle.fill" (the fill variant of calendar in SF Symbols)
- useColors: remove unsafe cast through Record<string, palette> —
  colors.radius (number) is incompatible with the palette shape;
  simplify to a direct ternary since both light and dark palettes
  are always defined

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

artifacts/postiz-mobile/app/(tabs)/_layout.tsx

@@ -13,7 +13,7 @@ function NativeTabLayout() {
   return (
     <NativeTabs>
       <NativeTabs.Trigger name="index">
-        <Icon sf={{ default: "calendar", selected: "calendar.fill" }} />
+        <Icon sf={{ default: "calendar", selected: "calendar.circle.fill" }} />
         <Label>Calendar</Label>
       </NativeTabs.Trigger>
       <NativeTabs.Trigger name="posts">