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>

Cargo.toml

@@ -9,6 +9,11 @@ repository = "https://github.com/your-username/tele-tui"
 keywords = ["telegram", "tui", "terminal", "cli"]
 categories = ["command-line-utilities"]
 
+[features]
+default = ["clipboard", "url-open"]
+clipboard = ["dep:arboard"]
+url-open = ["dep:open"]
+
 [dependencies]
 ratatui = "0.29"
 crossterm = "0.28"
@@ -18,8 +23,8 @@ serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 dotenvy = "0.15"
 chrono = "0.4"
-open = "5.0"
-arboard = "3.4"
+open = { version = "5.0", optional = true }
+arboard = { version = "3.4", optional = true }
 toml = "0.8"
 dirs = "5.0"
 thiserror = "1.0"