feat: seasonal race palette (Giro/Tour/Vuelta) + mobile picker

- theme.ts: useTheme() hook with race calendar (May–Sep windows),
  auto-detects Giro/Tour/Vuelta by date; stores override in SQLite
- All screens (feed, import, activity, tab bar) now use accent/dim
  from useTheme() instead of hardcoded #60a5fa
- Settings: Palette section with Auto/Default/Giro/Tour/Vuelta buttons
  to override the auto-detected palette for testing

mobile/app/(tabs)/_layout.tsx

@@ -1,12 +1,14 @@
 import { Tabs } from 'expo-router';
+import { useTheme } from '@/theme';
 
 export default function TabLayout() {
+  const theme = useTheme();
   return (
     <Tabs
       screenOptions={{
         headerShown: false,
         tabBarStyle: { backgroundColor: '#18181b', borderTopColor: '#27272a' },
-        tabBarActiveTintColor: '#60a5fa',
+        tabBarActiveTintColor: theme.accent,
         tabBarInactiveTintColor: '#71717a',
       }}
     >