merge: clear the clippy backlog and unify lock helpers (D1-warnings, D3)

51 clippy warnings -> 0, with 8 justified #[allow]s (IPC arity, specta wire
types, and the 9 test-only await-holding-lock sites). 27 raw lock calls moved to
the poison-tolerant helpers - all of them test code; production was already
clean.

Caught a non-neutral clippy --fix: removing the redundant 'use hostname;' in
credentials.rs orphaned its #[cfg(target_os = "linux")] onto SERVICE_NAME,
which would have cfg'd the constant out of every non-Linux build. Compiles clean
on Linux, so only Windows/macOS CI would have caught it.

src-tauri/src/lib.rs

@@ -620,7 +620,7 @@ fn create_player_backend(
         match MpvBackend::new(Some(_event_emitter), playback_reporter, position_throttler) {
             Ok(backend) => {
                 info!("Successfully initialized MPV backend for Linux");
-                return Box::new(backend);
+                Box::new(backend)
             }
             Err(e) => {
                 error!("\n========================================");
@@ -645,7 +645,7 @@ fn create_player_backend(
                 // still browse the library and manage downloads, and the frontend
                 // can show a "playback unavailable" notice via this event.
                 emit_backend_init_failed(&app_handle, "mpv", e.to_string());
-                return Box::new(NullBackend::new());
+                Box::new(NullBackend::new())
             }
         }
     }