feat: 热点穿透题材节点按涨幅正蓝负绿、球大小随涨幅绝对值

- 后端题材列表下发 bf3(题材涨幅)
- 题材节点: 正涨幅蓝色/负涨幅绿色, 半径按 |bf3| 在 6~20 映射
- 题材标签文字颜色跟随正负
- 信息卡/底部浮层题材分支显示涨幅
- 图例拆分涨/跌两项并说明球大小含义

Co-Authored-By: Claude <noreply@anthropic.com>

backend/services/themes.py

@@ -370,7 +370,7 @@ async def _build_theme_graph(sort_field: int, top_n: int) -> dict:
 
     return {
         "themes": [
-            {"themeCode": t["themeCode"], "themeName": t["themeName"], "stockCount": t["stockCount"]}
+            {"themeCode": t["themeCode"], "themeName": t["themeName"], "stockCount": t["stockCount"], "bf3": t.get("bf3")}
             for t in theme_map.values()
         ],
         "stocks": stocks,