refactor: unify error handling with anyhow and add From conversions

Phase 5: Replace all Box<dyn Error> return types with anyhow::Result<T>
throughout the codebase. Replace string-based Err("msg".into()) and
format!().into() patterns with bail!() and anyhow!() macros. Fix
dirs::home_dir().unwrap() in settings.rs to use a fallback path instead
of panicking when HOME is unset. Remove stray use std::error::Error
imports no longer needed.

Phase 6: Add From<&User> for CacheUser in models/user.rs and
From<&Laboratory>/From<&Laboratories> for CacheLaboratory/CacheLabsWrapper
in models/laboratory.rs. Simplify commands/login.rs to use .into()
conversions, removing the redundant to_cache_user() and to_cache_labs()
helper functions.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/api/users.rs

@@ -1,6 +1,7 @@
 use crate::connection::MDRSConnection;
 use crate::models::user::User as ModelUser;
 use serde::Deserialize;
+use anyhow::{bail};
 
 /// Full API response shape from GET v3/users/current/
 #[derive(Debug, Deserialize)]
@@ -40,7 +41,7 @@ impl MDRSConnection {
     pub async fn token_refresh(
         &self,
         refresh_token: &str,
-    ) -> Result<String, Box<dyn std::error::Error>> {
+    ) -> Result<String, anyhow::Error> {
         let body = serde_json::json!({ "refresh": refresh_token });
         let resp = self
             .client
@@ -49,7 +50,7 @@ impl MDRSConnection {
             .send()
             .await?;
         if !resp.status().is_success() {
-            return Err(format!("Token refresh failed: {}", resp.status()).into());
+            bail!("Token refresh failed: {}", resp.status());
         }
         let r: TokenRefreshResponse = resp.json().await?;
         Ok(r.access)