Fix Ctrl+C handling with proper tokio signal handler

Previous keyboard event polling didn't work for Ctrl+C in raw mode.
Now using tokio::signal::ctrl_c() with a background task that signals
the render loop through a channel when Ctrl+C is pressed.

Changes:
- Added "signal" and "sync" features to tokio dependency
- Spawn background task to listen for Ctrl+C signal
- Use mpsc channel to communicate signal to render loop
- Keep 'q' and ESC keyboard shortcuts for convenience

Ctrl+C now properly exits looping animations.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Cargo.toml

@@ -21,8 +21,8 @@ palette = "0.7"
 # Terminal manipulation
 crossterm = "0.27"
 
-# Async runtime (for timing)
-tokio = { version = "1.35", features = ["time", "rt-multi-thread", "macros"] }
+# Async runtime (for timing and signal handling)
+tokio = { version = "1.35", features = ["time", "rt-multi-thread", "macros", "signal", "sync"] }
 
 # Process execution
 which = "5.0"