chore(rust): clear the clippy backlog and finish the poison-tolerant lock sweep

`cargo clippy --all-targets` went from 51 warnings (23 in the lib) to zero.
Most were mechanical — needless borrows, `assert_eq!` against a bool literal,
`vec!` where an array does, `or_insert_with(Vec::new)`, a loop index used only
to index — and were applied with `clippy --fix`, then reviewed line by line.
That review caught one auto-fix that was *not* semantically neutral: dropping
the redundant `use hostname;` left its `#[cfg(target_os = "linux")]` orphaned
directly above `SERVICE_NAME`, which would have silently cfg'd the constant out
of every non-Linux build. Removed the stray attribute with the import.

Where a lint asked for a risky change rather than a better one, it is suppressed
with a comment saying why:

- `too_many_arguments` on five `#[tauri::command]` handlers and
  `ThumbnailCache::save_thumbnail` — most of the arity is `State<'_, _>`
  injection, and a parameter struct would change the IPC contract and the
  generated TypeScript for no readability gain.
- `large_enum_variant` on `PlayerStatusEvent` and `AutoplayDecision` — both are
  serde + specta wire types emitted a handful of times a second, never bulk
  allocated; boxing would have to stay invisible to the generated bindings while
  every match arm gained a deref.
- `await_holding_lock` on the `hybrid`/`offline` test modules — the guard is a
  test-only serialisation lock for the process-global `INCLUDE_CATALOG_BROWSE`
  flag, and the await it spans *is* the critical section. Each `#[tokio::test]`
  gets its own single-threaded runtime, so this is not the production deadlock
  class the lint targets; restructuring would reintroduce the flag race.

Real fixes elsewhere: `JellyfinItem::to_media_item` takes `self` by value, so it
is now `into_media_item`; the five-tuple episode row in the download commands
has a named `EpisodeRow` alias; the mpv `PropertyChange` arm matches
`name: "pause"` instead of guarding on it.

Also converted the last 27 raw `.lock().unwrap()` call sites to `lock_safe()`,
completing the `MutexSafe`/`RwLockSafe` convention. All of them turned out to be
in test modules — production code was already clean — so this is consistency
rather than a fix. The two raw locks in `utils/lock.rs` stay raw on purpose:
those tests deliberately poison a mutex to prove the helpers recover from it.

Pure refactoring: all 698 tests still pass.

src-tauri/src/thumbnail/cache.rs

@@ -155,6 +155,10 @@ impl ThumbnailCache {
     }
 
     /// Save thumbnail to cache
+    // The arguments are the cache key (item/type/tag) plus the payload and its
+    // dimensions — all independent scalars borrowed from the caller. A parameter
+    // struct would only move the same list one level down.
+    #[allow(clippy::too_many_arguments)]
     pub async fn save_thumbnail(
         &self,
         db: Arc<RusqliteService>,