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/components/BaseButton.tsx

@@ -0,0 +1,39 @@
+import { Pressable, Text } from "react-native";
+import { useThemeStore } from "../stores/ThemeStore";
+import { ReactNode } from "react";
+
+type BaseButtonProps = {
+  children?: ReactNode;
+  onPress: () => void;
+  solid?: boolean;
+};
+
+const BaseButton = ({children, onPress, solid = false}: BaseButtonProps) => {
+  const { theme } = useThemeStore();
+  return (
+    <Pressable
+      className="w-11/12 rounded-lg p-4 mb-4 border-4"
+      onPress={onPress}
+      style={{
+        borderColor: theme.borderPrimary,
+        backgroundColor: solid
+          ? theme.chatBot
+          : theme.primeBg,
+        shadowColor: theme.shadowColor,
+        shadowOffset: { width: 0, height: 2 },
+        shadowOpacity: 0.25,
+        shadowRadius: 3.84,
+        elevation: 5,
+      }}
+    >
+      <Text
+        className="text-center font-semibold text-lg"
+        style={{ color: theme.buttonText }}
+      >
+        {children}
+      </Text>
+    </Pressable>
+  );
+};
+
+export default BaseButton;