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/Themes.tsx

@@ -1,4 +1,4 @@
-type Theme = {
+export type Theme = {
   chatBot: string;
   primeFg: string;
   primeBg: string;
@@ -15,26 +15,46 @@ type Theme = {
   textMuted: string;
   eventIndicator: string;
   borderPrimary: string;
+  shadowColor: string;
 };
 
-const defaultLight: Theme = {
-  chatBot: "#DE6C20",
-  primeFg: "#3B3329",
-  primeBg: "#FFEEDE",
-  secondaryBg: "#FFFFFF",
-  messageBorderBg: "#FFFFFF",
-  placeholderBg: "#D9D9D9",
-  calenderBg: "#FBD5B2",
-  confirmButton: "#22c55e",
-  rejectButton: "#ef4444",
-  disabledButton: "#ccc",
-  buttonText: "#000000",
-  textPrimary: "#000000",
-  textSecondary: "#666",
-  textMuted: "#888",
-  eventIndicator: "#DE6C20",
-  borderPrimary: "#000000",
-};
-
-let currentTheme: Theme = defaultLight;
-export default currentTheme;
+export const THEMES = {
+  defaultLight: {
+    chatBot: "#DE6C20",
+    primeFg: "#3B3329",
+    primeBg: "#FFEEDE",
+    secondaryBg: "#FFFFFF",
+    messageBorderBg: "#FFFFFF",
+    placeholderBg: "#D9D9D9",
+    calenderBg: "#FBD5B2",
+    confirmButton: "#22c55e",
+    rejectButton: "#ef4444",
+    disabledButton: "#ccc",
+    buttonText: "#000000",
+    textPrimary: "#000000",
+    textSecondary: "#666",
+    textMuted: "#888",
+    eventIndicator: "#DE6C20",
+    borderPrimary: "#000000",
+    shadowColor: "#000000",
+  },
+  defaultDark: {
+    chatBot: "#DE6C20",
+    primeFg: "#F5E6D3",
+    primeBg: "#1A1512",
+    secondaryBg: "#2A2420",
+    messageBorderBg: "#3A3430",
+    placeholderBg: "#4A4440",
+    calenderBg: "#3D2A1A",
+    confirmButton: "#22c55e",
+    rejectButton: "#ef4444",
+    disabledButton: "#555",
+    buttonText: "#FFFFFF",
+    textPrimary: "#FFFFFF",
+    textSecondary: "#AAA",
+    textMuted: "#777",
+    eventIndicator: "#DE6C20",
+    borderPrimary: "#FFFFFF",
+    shadowColor: "#FFFFFF",
+  }
+} as const satisfies Record<string, Theme>;