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/BaseBackground.tsx

@@ -1,5 +1,5 @@
 import { View } from "react-native";
-import currentTheme from "../Themes";
+import { useThemeStore } from "../stores/ThemeStore";
 import { ReactNode } from "react";
 
 type BaseBackgroundProps = {
@@ -8,11 +8,12 @@ type BaseBackgroundProps = {
 };
 
 const BaseBackground = (props: BaseBackgroundProps) => {
+  const { theme } = useThemeStore();
   return (
     <View
       className={`h-full ${props.className}`}
       style={{
-        backgroundColor: currentTheme.primeBg,
+        backgroundColor: theme.primeBg,
       }}
     >
       {props.children}