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/utils/file_utils.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 
 import re
 from pathlib import Path
+from typing import Callable
 
 
 def get_server_dir(server_id: int) -> Path:
@@ -12,16 +13,27 @@ def get_server_dir(server_id: int) -> Path:
     return base / str(server_id)
 
 
-def ensure_server_dirs(server_id: int, layout: list[str] | None = None) -> None:
+def ensure_server_dirs(
+    server_id: int,
+    layout: list[str] | None = None,
+    readme_provider: Callable[[str], str | None] | None = None,
+) -> None:
     """
     Create servers/{id}/ and any subdirectories from adapter layout.
-    layout example: ["server", "battleye", "mpmissions"]
+    If readme_provider is given, writes README.txt into each subdir (skips if file already exists).
     """
     server_dir = get_server_dir(server_id)
     server_dir.mkdir(parents=True, exist_ok=True)
     if layout:
         for subdir in layout:
-            (server_dir / subdir).mkdir(parents=True, exist_ok=True)
+            subdir_path = server_dir / subdir
+            subdir_path.mkdir(parents=True, exist_ok=True)
+            if readme_provider:
+                content = readme_provider(subdir)
+                if content:
+                    readme_path = subdir_path / "README.txt"
+                    if not readme_path.exists():
+                        readme_path.write_text(content, encoding="utf-8")
 
 
 def safe_delete_file(path: Path) -> bool: