ci: make clippy a hard gate

The advisory step existed because the tree carried a warning backlog. Measured
on 1.97.1 — the pinned toolchain CI actually uses — that backlog is three
warnings, not the ~51 the comment claimed: two unnecessary_sort_by in
smart_cache and one redundant into_iter in offline. Fixed, so clippy now runs
with -D warnings and a warning means new breakage.

Worth recording why this took a toolchain pin to do safely: the same tree
measured 0 warnings on 1.92.0 and 3 on 1.97.1. Flipping the flag on a local
measurement, without the pin, would have reddened CI on the next push.

TRACES: | DR-206

src-tauri/src/repository/offline.rs

@@ -1128,7 +1128,7 @@ impl OfflineRepository {
         let device_total_bytes: i64 = leaves.iter().map(|(_, b)| *b).sum();
 
         let mut sizes = std::collections::HashMap::new();
-        for (id, bytes) in leaves.into_iter().chain(containers.into_iter()) {
+        for (id, bytes) in leaves.into_iter().chain(containers) {
             // A container id can never collide with a leaf id, so a plain insert
             // is fine; use entry to be defensive against duplicate rows.
             *sizes.entry(id).or_insert(0) += bytes;