fix: resolve all 40 clippy warnings (dead_code, unused_imports, lints)

- Add #[allow(unused_imports)] on pub re-exports used only by lib/tests
- Add #[allow(dead_code)] on public API items unused in binary target
- Fix collapsible_if, redundant_closure, unnecessary_map_or in main.rs
- Prefix unused test variables with underscore

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/accounts/mod.rs

@@ -7,5 +7,7 @@
 pub mod manager;
 pub mod profile;
 
+#[allow(unused_imports)]
 pub use manager::{add_account, ensure_account_dir, load_or_create, resolve_account, save};
+#[allow(unused_imports)]
 pub use profile::{account_db_path, validate_account_name, AccountProfile, AccountsConfig};