Fix chart viewport regressions from tail refresh and period switch.

Remove pendingViewportEpoch, fetch only 30 tail bars on poll, restore wasViewingTail logic, and fix left-scroll range shift from actual merge delta.

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

manual_trading_hub/hub.py

@@ -652,6 +652,7 @@ def api_chart_ohlcv(
     symbol: str = "",
     timeframe: str = "1d",
     refresh: str = "",
+    tail: str = "",
     limit: int = 0,
     before_ms: str = "",
 ):
@@ -665,6 +666,7 @@ def api_chart_ohlcv(
         raise HTTPException(status_code=400, detail="请输入币种")
     ex_key = str(ex.get("key") or "").strip().lower()
     force = (refresh or "").strip().lower() in ("1", "true", "yes", "on")
+    tail_refresh = (tail or "").strip().lower() in ("1", "true", "yes", "on")
     lim = int(limit) if int(limit or 0) > 0 else None
     bms_raw = (before_ms or "").strip()
     bms = None
@@ -690,6 +692,7 @@ def api_chart_ohlcv(
         timeframe,
         remote_fetch,
         force_refresh=force,
+        tail_refresh=tail_refresh,
         limit=lim,
         before_ms=bms,
     )