Add iOS Share Extension support

- Create share extension to handle shared content from other apps
- Support sharing text, URLs, and prepare for future image support
- Use app groups for data sharing between extension and main app
- Add URL scheme handling for app communication
- Add entitlements for app groups
- Display system message when content is shared
- Update project configuration to include share extension

bitchatShareExtension/bitchatShareExtension.entitlements

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>com.apple.security.app-sandbox</key>
+	<true/>
+	<key>com.apple.security.application-groups</key>
+	<array>
+		<string>group.chat.bitchat</string>
+	</array>
+</dict>
+</plist>