feat: 导航开关与 CTP 柜台手续费

系统设置可开关五类导航；手续费默认从 CTP 查询同步，本地/AKShare 作离线兜底；补充 FEES.md。

Co-authored-by: Cursor <cursoragent@cursor.com>

nav_settings.py

@@ -0,0 +1,46 @@
+"""顶栏导航项显示开关（系统设置）。"""
+from __future__ import annotations
+
+import json
+from typing import Callable
+
+# 可在系统设置中开关的导航项
+NAV_TOGGLES: dict[str, str] = {
+    "fees": "手续费配置",
+    "contract": "品种简介",
+    "plans": "开单计划",
+    "market": "行情K线",
+    "strategy": "策略交易",
+}
+
+DEFAULT_NAV: dict[str, bool] = {k: True for k in NAV_TOGGLES}
+
+
+def get_nav_items(get_setting: Callable[[str, str], str]) -> dict[str, bool]:
+    raw = (get_setting("nav_items", "") or "").strip()
+    out = dict(DEFAULT_NAV)
+    if not raw:
+        return out
+    try:
+        data = json.loads(raw)
+        if isinstance(data, dict):
+            for k in NAV_TOGGLES:
+                if k in data:
+                    out[k] = bool(data[k])
+    except json.JSONDecodeError:
+        pass
+    return out
+
+
+def save_nav_items(set_setting: Callable[[str, str], None], items: dict[str, bool]) -> None:
+    merged = dict(DEFAULT_NAV)
+    for k in NAV_TOGGLES:
+        if k in items:
+            merged[k] = bool(items[k])
+    set_setting("nav_items", json.dumps(merged, ensure_ascii=False))
+
+
+def nav_enabled(get_setting: Callable[[str, str], str], key: str) -> bool:
+    if key not in NAV_TOGGLES:
+        return True
+    return get_nav_items(get_setting).get(key, True)