feat: add EditEventScreen with calendar and chat mode support

Add a unified event editor that works in two modes:
- Calendar mode: Create/edit events directly via EventService API
- Chat mode: Edit AI-proposed events before confirming them

The chat mode allows users to modify proposed events (title, time,
recurrence) and persists changes both locally and to the server.

New components: DateTimePicker, ScrollableDropdown, useDropdownPosition
New API: PUT /api/chat/messages/:messageId/proposal

apps/client/src/app/(tabs)/settings.tsx

@@ -5,7 +5,7 @@ import { useThemeStore } from "../../stores/ThemeStore";
 import { AuthService } from "../../services/AuthService";
 import { router } from "expo-router";
 import { Ionicons } from "@expo/vector-icons";
-import Header from "../../components/Header";
+import { SimpleHeader } from "../../components/Header";
 import { THEMES } from "../../Themes";
 
 const handleLogout = async () => {
@@ -18,11 +18,7 @@ const Settings = () => {
 
   return (
     <BaseBackground>
-      <Header>
-        <View className="h-full flex justify-center">
-          <Text className="text-center text-3xl font-bold">Settings</Text>
-        </View>
-      </Header>
+      <SimpleHeader text="Settings" />
       <View className="flex items-center mt-4">
         <BaseButton onPress={handleLogout} solid={true}>
           <Ionicons name="log-out-outline" size={24} color={theme.primeFg} />{" "}