feat: Add erase button to clear chat messages

Add trash icon button in chat header that clears all messages
after confirmation. Resets chat to initial welcome message.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/(tabs)/chat.tsx

@@ -732,7 +732,7 @@ export default function ChatScreen() {
         <View style={[styles.messageBubble, isUser ? styles.userBubble : styles.assistantBubble, isVoice && styles.voiceBubble]}>
           {isVoice && (
             <View style={styles.voiceIndicator}>
-              <Ionicons name="mic" size={12} color={isUser ? 'rgba(255,255,255,0.7)' : AppColors.textMuted} />
+              <Text style={styles.voiceIndicatorEmoji}>🎤</Text>
             </View>
           )}
           <Text style={[styles.messageText, isUser ? styles.userMessageText : styles.assistantMessageText]}>
@@ -764,7 +764,34 @@ export default function ChatScreen() {
             </Text>
           </View>
         </View>
-        <View style={styles.headerButtons} />
+        <TouchableOpacity
+          style={styles.eraseButton}
+          onPress={() => {
+            Alert.alert(
+              'Clear Chat',
+              'Are you sure you want to clear all messages?',
+              [
+                { text: 'Cancel', style: 'cancel' },
+                {
+                  text: 'Clear',
+                  style: 'destructive',
+                  onPress: () => {
+                    setMessages([
+                      {
+                        id: '1',
+                        role: 'assistant',
+                        content: 'Hello! I\'m Julia, your AI wellness assistant. You can type a message or tap the phone button to start a voice call.',
+                        timestamp: new Date(),
+                      },
+                    ]);
+                  },
+                },
+              ]
+            );
+          }}
+        >
+          <Ionicons name="trash-outline" size={22} color={AppColors.textSecondary} />
+        </TouchableOpacity>
       </View>
 
       {/* Beneficiary Picker Modal */}
@@ -972,12 +999,7 @@ const styles = StyleSheet.create({
     fontSize: FontSizes.sm,
     color: AppColors.success,
   },
-  headerButtons: {
-    flexDirection: 'row',
-    alignItems: 'center',
-    gap: Spacing.sm,
-  },
-  headerButton: {
+  eraseButton: {
     padding: Spacing.xs,
   },
   chatContainer: {
@@ -1175,6 +1197,9 @@ const styles = StyleSheet.create({
     top: 6,
     right: 6,
   },
+  voiceIndicatorEmoji: {
+    fontSize: 10,
+  },
   // System message styles
   systemMessageContainer: {
     flexDirection: 'row',