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/attachments.rs

@@ -68,7 +68,7 @@ fn detach_removes_attachment_and_blob() {
     // Encrypted blob file is gone.
     let blob_path = v.path()
         .join("attachments")
-        .join(stdout.lines().nth(1).is_some().then_some("").unwrap_or(""));
+        .join("");
     let item_attach_dir = std::fs::read_dir(v.path().join("attachments"))
         .unwrap().next().unwrap().unwrap().path();
     let blob = item_attach_dir.join(format!("{aid}.enc"));

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())