feat: add optional feature flags for clipboard and url-open (P5.15)

Implemented feature flags to make dependencies optional:
- clipboard feature: controls arboard dependency
- url-open feature: controls open dependency
- Both enabled by default for backward compatibility

Changes:
- Cargo.toml: Added [features] section with optional deps
- src/input/main_input.rs: Conditional compilation for open::that() and copy_to_clipboard()
- tests/copy.rs: Clipboard tests only compile with feature enabled
- Graceful degradation: user-friendly error messages when features disabled

Benefits:
- Smaller binary size when features disabled
- Modular functionality
- Better platform compatibility

Progress: Priority 5: 1/3 tasks, Total: 17/20 (85%)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/input/main_input.rs

@@ -138,15 +138,24 @@ pub async fn handle(app: &mut App, key: KeyEvent) {
                                         "https://t.me/{}",
                                         username.trim_start_matches('@')
                                     );
-                                    match open::that(&url) {
-                                        Ok(_) => {
-                                            app.status_message = Some(format!("Открыто: {}", url));
-                                        }
-                                        Err(e) => {
-                                            app.error_message =
-                                                Some(format!("Ошибка открытия браузера: {}", e));
+                                    #[cfg(feature = "url-open")]
+                                    {
+                                        match open::that(&url) {
+                                            Ok(_) => {
+                                                app.status_message = Some(format!("Открыто: {}", url));
+                                            }
+                                            Err(e) => {
+                                                app.error_message =
+                                                    Some(format!("Ошибка открытия браузера: {}", e));
+                                            }
                                         }
                                     }
+                                    #[cfg(not(feature = "url-open"))]
+                                    {
+                                        app.error_message = Some(
+                                            "Открытие URL недоступно (требуется feature 'url-open')".to_string()
+                                        );
+                                    }
                                 }
                                 return;
                             }
@@ -1057,6 +1066,7 @@ fn get_available_actions_count(profile: &crate::tdlib::ProfileInfo) -> usize {
 }
 
 /// Копирует текст в системный буфер обмена
+#[cfg(feature = "clipboard")]
 fn copy_to_clipboard(text: &str) -> Result<(), String> {
     use arboard::Clipboard;
 
@@ -1069,6 +1079,12 @@ fn copy_to_clipboard(text: &str) -> Result<(), String> {
     Ok(())
 }
 
+/// Заглушка для copy_to_clipboard когда feature "clipboard" выключена
+#[cfg(not(feature = "clipboard"))]
+fn copy_to_clipboard(_text: &str) -> Result<(), String> {
+    Err("Копирование в буфер обмена недоступно (требуется feature 'clipboard')".to_string())
+}
+
 /// Форматирует сообщение для копирования с контекстом
 fn format_message_for_clipboard(msg: &crate::tdlib::MessageInfo) -> String {
     let mut result = String::new();