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-core/tests/safe_unpack.rs

@@ -51,12 +51,12 @@ fn raw_tar_with_path(raw_path: &[u8], content: &[u8]) -> Vec<u8> {
         // Pad to 512-byte boundary.
         let remainder = content.len() % 512;
         if remainder != 0 {
-            out.extend(std::iter::repeat(0u8).take(512 - remainder));
+            out.extend(vec![0u8; 512 - remainder]);
         }
     }
 
     // Two zero blocks = end-of-archive.
-    out.extend(std::iter::repeat(0u8).take(1024));
+    out.extend(vec![0u8; 1024]);
     out
 }
 
@@ -92,7 +92,7 @@ fn raw_symlink_tar() -> Vec<u8> {
     buf[148..156].copy_from_slice(cksum_str.as_bytes());
 
     let mut out = buf;
-    out.extend(std::iter::repeat(0u8).take(1024)); // end-of-archive
+    out.extend(vec![0u8; 1024]); // end-of-archive
     out
 }