fix: handle Ctrl+C setup failure gracefully

[echobt]

Description

The file watcher previously used .expect() when setting up the Ctrl+C handler, which could cause a panic in environments where signal handlers cannot be set (e.g., non-TTY contexts, containers). This PR changes the implementation to handle the error gracefully by logging a warning and continuing execution.

Changes

• Replaced ctrlc::set_handler(...).expect(...) with if let Err(e) = ctrlc::set_handler(...) { eprintln!(...); }
• Logs a warning message if the signal handler cannot be installed, allowing the application to continue running.

Verification

• Manual verification: Ran vgrep watch < /dev/null to ensure no regression in normal operation.
• Code inspection: Verified that the failure path no longer panics.