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/src/time.rs

@@ -19,7 +19,7 @@ impl MonthYear {
         if !(1..=12).contains(&month) {
             return Err("month must be 1..=12");
         }
-        if year < 2000 || year > 2099 {
+        if !(2000..=2099).contains(&year) {
             return Err("year must be 2000..=2099");
         }
         Ok(Self { month, year })