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>

tests/copy.rs

@@ -111,7 +111,7 @@ fn format_message_for_test(msg: &tele_tui::tdlib::MessageInfo) -> String {
     result
 }
 
-#[cfg(test)]
+#[cfg(all(test, feature = "clipboard"))]
 mod clipboard_tests {
     use super::*;