refactor(main): use GimError exit codes instead of implicit panics

main() now calls gim::run(args) which returns Result, and maps
errors to stderr output with the correct exit code from GimError.
No more process::exit() calls buried in library code.

Author: dev-dami

src/main.rs

@@ -1,3 +1,10 @@
+use clap::Parser;
+
 fn main() {
-    gim::run();
+    let args = gim::cli::Cli::parse();
+
+    if let Err(e) = gim::run(args) {
+        eprintln!("error: {e}");
+        std::process::exit(e.exit_code());
+    }
 }