feat: Add comprehensive improvements - async optimization, caching, and error handling

- Add async optimization using futures for concurrent file processing
- Implement intelligent caching system with SHA256 file change detection
- Add comprehensive custom error handling for Gemini API responses
- Fix critical error handling issues throughout the codebase
- Replace fragile JSON parsing with proper struct-based deserialization
- Add automatic retry logic for rate limits and network issues
- Improve user experience with detailed error messages and progress feedback
- Add cache persistence and automatic cleanup of old entries
- Optimize performance for batch processing scenarios

Cargo.toml

@@ -4,8 +4,15 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
+colored = "3.0.0"
 dotenv = "0.15.0"
+futures = "0.3.31"
+hex = "0.4.3"
+itertools = "0.14.0"
 reqwest = { version = "0.12.26", features = ["json"] }
 serde = { version = "1.0.228", features = ["derive"] }
 serde_json = "1.0.145"
+sha2 = "0.10.8"
+thiserror = "2.0.11"
 tokio = { version = "1.48.0", features = ["full"] }
+walkdir = "2.5.0"