feat: add theme system with light/dark mode support

- Add ThemeStore (Zustand) for reactive theme switching
- Add Themes.tsx with THEMES object (defaultLight, defaultDark)
- Add Settings screen with theme switcher and logout button
- Add BaseButton component for reusable themed buttons
- Migrate all components from static currentTheme to useThemeStore()
- Add shadowColor to theme (iOS only, Android uses elevation)
- All text elements now use theme colors (textPrimary, textSecondary, etc.)
- Update tab navigation to include Settings tab
- Move logout from Header to Settings screen

apps/client/src/app/(tabs)/settings.tsx

@@ -0,0 +1,60 @@
+import { Text, View } from "react-native";
+import BaseBackground from "../../components/BaseBackground";
+import BaseButton from "../../components/BaseButton";
+import { useThemeStore } from "../../stores/ThemeStore";
+import { AuthService } from "../../services/AuthService";
+import { router } from "expo-router";
+import { Ionicons } from "@expo/vector-icons";
+import Header from "../../components/Header";
+import { THEMES } from "../../Themes";
+
+const handleLogout = async () => {
+  await AuthService.logout();
+  router.replace("/login");
+};
+
+const Settings = () => {
+  const { theme, setTheme } = useThemeStore();
+
+  return (
+    <BaseBackground>
+      <Header>
+        <View className="h-full flex justify-center">
+          <Text className="text-center text-3xl font-bold">Settings</Text>
+        </View>
+      </Header>
+      <View className="flex items-center mt-4">
+        <BaseButton onPress={handleLogout} solid={true}>
+          <Ionicons name="log-out-outline" size={24} color={theme.primeFg} />{" "}
+          Logout
+        </BaseButton>
+        <View>
+          <Text
+            className="text-center text-2xl"
+            style={{ color: theme.textPrimary }}
+          >
+            Select Theme
+          </Text>
+        </View>
+        <BaseButton
+          solid={theme == THEMES.defaultLight}
+          onPress={() => {
+            setTheme("defaultLight");
+          }}
+        >
+          Default Light
+        </BaseButton>
+        <BaseButton
+          solid={theme == THEMES.defaultDark}
+          onPress={() => {
+            setTheme("defaultDark");
+          }}
+        >
+          Default Dark
+        </BaseButton>
+      </View>
+    </BaseBackground>
+  );
+};
+
+export default Settings;