avoid unsafe unwrapping

Replace .unwrap() calls and sentinel values with safe null handling patterns to prevent runtime panics and improve code robustness.

Key Issues:

1. Unsafe unwrapping: Using .unwrap() on operations that could fail, especially downcasts and IPC operations
2. Sentinel values: Using invalid constants like Invalid or None = -1 instead of proper Option types
3. Missing graceful degradation: Not handling None cases appropriately

Safe Patterns:

• Use if let Some(value) = optional_value instead of .unwrap()
• Use .expect("meaningful message") when unwrapping is truly safe
• Replace sentinel values with Option<T> types
• Use ? operator for early returns with Options
• Return Option<T> instead of default values when absence is meaningful

Example:

// ❌ Unsafe - could panic
let video_elem = self.downcast::<HTMLVideoElement>().unwrap();
video_elem.resize(width, height);

// ✅ Safe pattern
if let Some(video_elem) = self.downcast::<HTMLVideoElement>() {
    video_elem.resize(width, height);
}

// ❌ Sentinel value antipattern  
pub enum LargestContentfulPaintType {
    None = -1,
    // ...
}

// ✅ Proper Option usage
pub fn get_lcp_candidate() -> Option<LCPCandidate> {
    // ...
}

This prevents runtime crashes and makes null states explicit in the type system, improving both safety and code clarity.