feat(backend): add thiserror-based error handling

Introduce AppError enum to replace panic-based error handling in executable loading and state management. Adds proper error propagation with descriptive error messages for missing executables, key patterns, and environment variables.

backend/src/lib.rs

@@ -1,4 +1,5 @@
 pub mod config;
+pub mod errors;
 pub mod handlers;
 pub mod models;
 pub mod railway;