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/adapters/arma3/process_config.py

@@ -27,4 +27,50 @@ class Arma3ProcessConfig:
 
     def get_server_dir_layout(self) -> list[str]:
         """Subdirectories to create inside servers/{id}/."""
-        return ["server", "battleye", "mpmissions"]
+        return ["server", "battleye", "mpmissions", "mods"]
+
+    _DIR_READMES: dict[str, str] = {
+        "server": (
+            "Arma 3 Server — Log Directory\n"
+            "==============================\n\n"
+            "Arma 3 writes RPT log files here (e.g. arma3server_2024-01-01_12-00-00.rpt).\n"
+            "These are viewable in Languard's Logs tab.\n\n"
+            "Do NOT place files here manually."
+        ),
+        "battleye": (
+            "BattlEye Anti-Cheat\n"
+            "===================\n\n"
+            "BattlEye configuration and GUID ban list files live here.\n"
+            "Managed automatically by Arma 3 and Languard.\n\n"
+            "Do NOT modify these files manually unless you know what you are doing."
+        ),
+        "mpmissions": (
+            "Mission Files\n"
+            "=============\n\n"
+            "Place Arma 3 mission files (.pbo) here to make them available for the server.\n"
+            "Once placed here they will appear in Languard's Missions tab.\n\n"
+            "Example: Wasteland_A3.Altis.pbo"
+        ),
+        "mods": (
+            "Mods\n"
+            "====\n\n"
+            "Place Arma 3 mod folders here. Each mod folder must start with '@'.\n\n"
+            "Example layout:\n"
+            "  mods/\n"
+            "    @CBA_A3/\n"
+            "      addons/\n"
+            "    @ACE/\n"
+            "      addons/\n\n"
+            "After placing mods here:\n"
+            "  1. Go to the Mods tab in Languard.\n"
+            "  2. Select the mods you want to enable.\n"
+            "  3. Toggle 'Server-only' for mods that should use -serverMod= (e.g. task force radio server plugin).\n"
+            "  4. Click 'Apply Selection'.\n"
+            "  5. Restart the server for changes to take effect.\n\n"
+            "Mods with a mod.cpp file will display their friendly name in the UI.\n"
+            "Workshop mods with meta.cpp will show their Workshop ID."
+        ),
+    }
+
+    def get_dir_readme(self, dir_name: str) -> str | None:
+        return self._DIR_READMES.get(dir_name)