fix(plugins): silence plugin warnings and folder creation when plugins disabled (#4297)

* fix(plugins): silence repeated “Plugin not found” spam for inactive Spotify/Last.fm plugins

Navidrome was emitting a warning when the optional Spotify or
Last.fm agents weren’t enabled, filling the journal with entries like:

    level=warning msg="Plugin not found" capability=MetadataAgent name=spotify

Fixed by completely disable the plugin system when Plugins.Enabled = false.

Signed-off-by: Deluan <deluan@navidrome.org>

* style: update test description for clarity

Signed-off-by: Deluan <deluan@navidrome.org>

* fix: ensure plugin folder is created only if plugins are enabled

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

plugins/host_websocket_test.go

@@ -17,7 +17,7 @@ import (
 var _ = Describe("WebSocket Host Service", func() {
 	var (
 		wsService      *websocketService
-		manager        *Manager
+		manager        *managerImpl
 		ctx            context.Context
 		server         *httptest.Server
 		upgrader       gorillaws.Upgrader