feat: fix mods tab, add client/server split, and scaffold server dirs

Mods tab bug fixes:
- mod_manager: fix wrong kwargs in set_enabled_mods, fix scan dir to use
  mods/ subdir instead of server root, migrate old string-list format to
  dict format on read
- service: replace dead server_mods SQL JOIN with get_enabled_mods()
  call through the mod_manager capability; pass is_server_mod to
  build_mod_args
- mods_router: accept list[EnabledModEntry] objects (name + is_server_mod)
  instead of bare strings

Client/server mod split:
- Mods now stored as list[{"name": str, "is_server_mod": bool}]; old
  string-list format auto-migrated on read
- is_server_mod=true routes to -serverMod= arg; false to -mod= arg
- ModList UI: amber Client/Server badge in selected pane; toggle button
  in split-pane selector

Directory scaffold:
- process_config: adds "mods" to dir layout; provides get_dir_readme()
  with per-directory README.txt content
- file_utils: ensure_server_dirs() gains readme_provider kwarg; writes
  README.txt idempotently if absent
- service.create_server: passes readme_provider via hasattr probe
- main.py startup: backfills all existing servers with correct subdirs
  and README files (idempotent)

Docs: API.md and FRONTEND.md updated for new mod schema and types
Test __init__.py files added for pytest discovery

backend/core/servers/service.py

@@ -126,9 +126,10 @@ class ServerService:
             max_restarts=max_restarts,
         )
 
-        # Create directory layout
+        # Create directory layout with per-directory README files
         layout = process_config.get_server_dir_layout()
-        ensure_server_dirs(server_id, layout)
+        readme_fn = getattr(process_config, "get_dir_readme", None)
+        ensure_server_dirs(server_id, layout, readme_provider=readme_fn)
 
         # Seed default config sections
         config_gen = adapter.get_config_generator()
@@ -242,17 +243,17 @@ class ServerService:
         # Get mod args if adapter supports mods
         mod_args: list[str] = []
         if adapter.has_capability("mod_manager"):
-            from sqlalchemy import text
-            mods = self._db.execute(
-                text("""
-                SELECT m.folder_path, sm.is_server_mod, sm.sort_order
-                FROM server_mods sm JOIN mods m ON m.id = sm.mod_id
-                WHERE sm.server_id = :sid ORDER BY sm.sort_order
-                """),
-                {"sid": server_id},
-            ).fetchall()
-            mod_list = [dict(r._mapping) for r in mods]
-            mod_args = adapter.get_mod_manager().build_mod_args(mod_list)
+            mod_mgr = adapter.get_mod_manager(server_id)
+            enabled_mods = mod_mgr.get_enabled_mods(self._config_repo)
+            server_dir = get_server_dir(server_id)
+            mod_list = [
+                {
+                    "folder_path": str(server_dir / "mods" / m["name"]),
+                    "game_data": {"is_server_mod": m.get("is_server_mod", False)},
+                }
+                for m in enabled_mods
+            ]
+            mod_args = mod_mgr.build_mod_args(mod_list)
 
         # Write config files (atomic)
         server_dir = get_server_dir(server_id)