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/helpers.rs

@@ -34,6 +34,7 @@ pub fn vault_dir() -> Result<PathBuf> {
 }
 
 /// Path to the `.relicario/` configuration directory within the vault.
+#[allow(dead_code)]
 pub fn relicario_dir() -> Result<PathBuf> {
     Ok(vault_dir()?.join(".relicario"))
 }