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/stores/ThemeStore.ts

@@ -0,0 +1,12 @@
+import { create } from "zustand";
+import { Theme, THEMES } from "../Themes";
+
+interface ThemeState {
+  theme: Theme;
+  setTheme: (themeName: keyof typeof THEMES) => void;
+}
+
+export const useThemeStore = create<ThemeState>((set) => ({
+  theme: THEMES.defaultLight,
+  setTheme: (themeName) => set({theme: THEMES[themeName]})
+}))