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/tests/common/mod.rs

@@ -78,6 +78,7 @@ impl TestVault {
         cmd.output().unwrap()
     }
 
+    #[allow(dead_code)]
     pub fn run_with_backup_pass(&self, args: &[&str], backup_pass: &str) -> std::process::Output {
         let mut cmd = Command::cargo_bin("relicario").unwrap();
         cmd.current_dir(self.dir.path())
@@ -91,6 +92,7 @@ impl TestVault {
         cmd.output().unwrap()
     }
 
+    #[allow(dead_code)]
     pub fn run_with_input(&self, args: &[&str], extra: &[&str]) -> std::process::Output {
         let mut cmd = Command::cargo_bin("relicario").unwrap();
         cmd.current_dir(self.dir.path())