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/mods_router.py

@@ -24,8 +24,13 @@ def _ok(data):
     return {"success": True, "data": data, "error": None}
 
 
+class EnabledModEntry(BaseModel):
+    name: str
+    is_server_mod: bool = False
+
+
 class SetEnabledModsRequest(BaseModel):
-    mods: list[str]
+    mods: list[EnabledModEntry]
 
 
 def _get_mod_manager(server_id: int, game_type: str):
@@ -52,12 +57,15 @@ def list_mods(
     config_repo = ConfigRepository(db)
     try:
         available = mgr.list_available_mods()
-        enabled = set(mgr.get_enabled_mods(config_repo))
+        enabled_mods = mgr.get_enabled_mods(config_repo)
     except AdapterError as exc:
         raise HTTPException(status_code=500, detail={"code": "ADAPTER_ERROR", "message": str(exc)})
 
+    enabled_map = {m["name"]: m for m in enabled_mods}
     for mod in available:
-        mod["enabled"] = mod["name"] in enabled
+        entry = enabled_map.get(mod["name"])
+        mod["enabled"] = entry is not None
+        mod["is_server_mod"] = entry["is_server_mod"] if entry else False
 
     return _ok({
         "server_id": server_id,
@@ -83,7 +91,7 @@ def set_enabled_mods(
 
     config_repo = ConfigRepository(db)
     try:
-        mgr.set_enabled_mods(body.mods, config_repo)
+        mgr.set_enabled_mods([m.model_dump() for m in body.mods], config_repo)
     except AdapterError as exc:
         raise HTTPException(status_code=400, detail={"code": "ADAPTER_ERROR", "message": str(exc)})
     except ValueError as exc:
@@ -97,5 +105,5 @@ def set_enabled_mods(
 
     return _ok({
         "message": "Enabled mods updated. Restart the server for changes to take effect.",
-        "enabled_mods": body.mods,
+        "enabled_mods": [m.model_dump() for m in body.mods],
     })