fix(home): server-owned growth window; expose days

Home omits growth_days; backend auto (or home_growth_days 1/7) picks
the window. Response includes days alongside growth_days. Growth榜 link
uses the returned days. Non-growth sorts treat omitted/auto as 7.

reaction_board/web/server.py

@@ -650,11 +650,10 @@ def _year_param(query: Mapping[str, list[str]]) -> int | None:
 
 
 def _growth_days_param(query: Mapping[str, list[str]]) -> int | str:
-    """榜单传 1~365；首页可传 auto（由查询层按 home_growth_days 解析）。"""
+    """省略或 auto：查询层按 home_growth_days 解析（默认 7→1）。
+    榜单应显式传 1~365。"""
     raw = _optional_str(query, "growth_days")
-    if raw is None:
-        return 7
-    if raw == "auto":
+    if raw is None or raw == "auto":
         return "auto"
     try:
         value = int(raw)