chore(core): fix pre-existing clippy warnings (-D warnings gate)

Resolves pre-existing lint issues in imgsecret.rs, time.rs, totp.rs,
and crypto.rs that blocked the cargo clippy --workspace -D warnings
gate. No logic changes: loop-index → iterator, manual div_ceil →
.div_ceil(), manual range contains → .contains(), auto-deref cleanup.

Also fixes pre-existing warnings in relicario-cli (main.rs, session.rs,
device.rs, gitea.rs, helpers.rs, test helpers): dead_code suppression,
too_many_arguments, literal_with_empty_format_string, manual_char_cmp,
map_or → is_none_or, and repeat().take() → vec! in test helpers.

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

crates/relicario-cli/src/gitea.rs

@@ -21,7 +21,9 @@ struct CreateKeyRequest<'a> {
 #[derive(Debug, Deserialize)]
 pub struct DeployKey {
     pub id: u64,
+    #[allow(dead_code)]
     pub title: String,
+    #[allow(dead_code)]
     pub key: String,
 }
 
@@ -89,6 +91,7 @@ impl GiteaClient {
     }
 
     /// List all deploy keys.
+    #[allow(dead_code)]
     pub fn list_deploy_keys(&self) -> Result<Vec<DeployKey>> {
         let url = format!(
             "{}/repos/{}/{}/keys",