🎨 Palette: Enhance anyhow nested error formatting in CLI

.Jules/palette.md

@@ -1,3 +1,14 @@
 ## 2024-07-11 - Route UI print methods to stderr
 **Learning:** Routing standard informational/success/error/warning messages to `stderr` allows users to pipe CLI data from `stdout` cleanly without mixing it with UI feedback. This is a common pattern in Unix tools.
 **Action:** Ensure `ui.print_*` methods use `eprintln!` instead of `println!`.
+## 2024-07-18 - Better nested errors with anyhow's chain
+
+**Learning:** anyhow's default `Debug` format displays errors as a raw numbered list like:
+```
+Caused by:
+    0: Failed to send login request
+    1: client error
+```
+Which is not very pleasing to the eye or easy to parse for users.
+
+**Action:** Instead of returning `anyhow::Result<()>` from `main()`, we can return `std::process::ExitCode`. This allows us to manually iterate through `anyhow::Error::chain().enumerate()`, displaying the root cause in bold, and cleanly indenting nested causes with a `↳` symbol.

src/main.rs

@@ -1,15 +1,28 @@
-use anyhow::Result;
 use clap::Parser;
+use console::style;
 use kaji::args::Cli;
+use kaji::utils::ui::ERROR;
 
 #[cfg(not(tarpaulin_include))]
 #[tokio::main]
-async fn main() -> Result<()> {
+async fn main() -> std::process::ExitCode {
     dotenvy::dotenv().ok();
     dotenvy::from_filename(".secrets").ok();
     env_logger::init();
 
     let cli = Cli::parse();
 
-    kaji::run_app(cli).await
+    if let Err(err) = kaji::run_app(cli).await {
+        eprintln!("{} {}", ERROR, style("Error:").red().bold());
+        for (i, cause) in err.chain().enumerate() {
+            if i == 0 {
+                eprintln!("  {}", style(cause).bold());
+            } else {
+                eprintln!("    {} {}", style("↳").dim(), cause);
+            }
+        }
+        std::process::ExitCode::FAILURE
+    } else {
+        std::process::ExitCode::SUCCESS
+    }
 }