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/ChatBubble.tsx

@@ -1,5 +1,5 @@
 import { View, ViewStyle } from "react-native";
-import colors from "../Themes";
+import { useThemeStore } from "../stores/ThemeStore";
 
 type BubbleSide = "left" | "right";
 
@@ -11,7 +11,8 @@ type ChatBubbleProps = {
 };
 
 export function ChatBubble({ side, children, className = "", style }: ChatBubbleProps) {
-  const borderColor = side === "left" ? colors.chatBot : colors.primeFg;
+  const { theme } = useThemeStore();
+  const borderColor = side === "left" ? theme.chatBot : theme.primeFg;
   const sideClass =
     side === "left"
       ? "self-start ml-2 rounded-bl-sm"
@@ -19,8 +20,8 @@ export function ChatBubble({ side, children, className = "", style }: ChatBubble
 
   return (
     <View
-      className={`bg-white border-2 border-solid rounded-xl my-2 ${sideClass} ${className}`}
-      style={[{ borderColor, elevation: 8 }, style]}
+      className={`border-2 border-solid rounded-xl my-2 ${sideClass} ${className}`}
+      style={[{ borderColor, elevation: 8, backgroundColor: theme.secondaryBg }, style]}
     >
       {children}
     </View>