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/cache/types.rs

@@ -0,0 +1,42 @@
+use serde::Deserialize;
+
+/// Access and refresh token pair stored in the login cache file.
+#[derive(Deserialize, Clone)]
+pub struct CacheToken {
+    pub access: String,
+    pub refresh: String,
+}
+
+/// Minimal user fields stored in the cache, matching Python's `User` dataclass.
+#[derive(Deserialize, Clone)]
+pub struct CacheUser {
+    pub id: u32,
+    pub username: String,
+    pub laboratory_ids: Vec<u32>,
+    pub is_reviewer: bool,
+}
+
+/// All four laboratory fields needed for digest computation.
+#[derive(Deserialize, Clone)]
+pub struct CacheLaboratory {
+    pub id: u32,
+    pub name: String,
+    #[serde(default)]
+    pub pi_name: String,
+    #[serde(default)]
+    pub full_name: String,
+}
+
+/// Wrapper matching Python's `Laboratories` serialization: `{"items": [...]}`.
+#[derive(Deserialize, Clone, Default)]
+pub struct CacheLabsWrapper {
+    pub items: Vec<CacheLaboratory>,
+}
+
+/// Full login cache, corresponding to the `<remote>.json` file written by `login`.
+#[derive(Deserialize, Clone)]
+pub struct Cache {
+    pub user: Option<CacheUser>,
+    pub token: CacheToken,
+    pub laboratories: CacheLabsWrapper,
+}