fix: palette changes now propagate immediately via ThemeContext

Replace useSetting()-based useTheme() with a React context (ThemeProvider
+ useTheme/usePaletteControl). The context holds palette key in state so
pressing a palette button in Settings re-renders all screens instantly.
Persists to SQLite and reloads the stored value on mount.

mobile/app/_layout.tsx

@@ -2,12 +2,15 @@ import { Stack } from 'expo-router';
 import { SQLiteProvider } from 'expo-sqlite';
 import { StatusBar } from 'expo-status-bar';
 import { migrateDb } from '@/db';
+import { ThemeProvider } from '@/ThemeContext';
 
 export default function RootLayout() {
   return (
     <SQLiteProvider databaseName="bincio.db" onInit={migrateDb}>
-      <StatusBar style="light" />
-      <Stack screenOptions={{ headerShown: false }} />
+      <ThemeProvider>
+        <StatusBar style="light" />
+        <Stack screenOptions={{ headerShown: false }} />
+      </ThemeProvider>
     </SQLiteProvider>
   );
 }