feat: implement phases 3-5 of Arma 3 UX enhancement plan

Phase 3 - Mod display names + split-pane selector:
- Parse mod.cpp/meta.cpp for display_name and workshop_id
- Rewrite ModList as two-pane available/selected interface

Phase 4 - Player kick/ban from Players tab:
- Add get_by_slot() to PlayerRepository
- Add get_rcon_client() class method to ThreadRegistry
- Add /players/{slot_id}/kick and /ban endpoints
- Rewrite PlayerTable with kick/ban modals and ban presets

Phase 5 - Historical log file browser:
- Add list_log_files() and get_log_file_path() to RPTParser
- Add logfiles_router with GET/download/DELETE endpoints
- Update LogViewer with collapsible log files section (download + delete)

backend/core/threads/thread_registry.py

@@ -90,6 +90,20 @@ class ThreadRegistry:
         if registry is not None:
             registry._stop_all()
 
+    @classmethod
+    def get_rcon_client(cls, server_id: int):
+        """Return the live Arma3RemoteAdmin client for a running server, or None."""
+        registry = cls._get_instance()
+        if registry is None:
+            return None
+        bundle = registry._bundles.get(server_id)
+        if bundle is None:
+            return None
+        poller = bundle.get("rcon_poller")
+        if poller is None or not poller.is_alive():
+            return None
+        return getattr(poller, "_client", None)
+
     # ── Instance methods ──
 
     def _start_server_threads(self, server_id: int, db) -> None: