fix: don't fabricate last_equity for Tradier — realized P&L read $0 forever

Dashboard realized = broker equity delta (equity - last_equity). Tradier
balances carry no prior-session-close field and translate_balances echoed
current equity, making the delta permanently zero (win rate populated from
closed events while dollars showed $0). Absent last_equity now routes the
projection to its closed-events fallback.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tradier_broker/client.py

@@ -164,7 +164,13 @@ def translate_balances(b: dict) -> dict:
     )
     return {
         "equity": _s(b.get("total_equity") or 0),
-        "last_equity": _s(b.get("total_equity") or 0),
+        # Tradier has NO prior-session-close equity field. Do NOT fake it with
+        # current equity: the dashboard's realized P&L is the broker equity
+        # delta (equity - last_equity), which then reads $0 forever. Absent
+        # last_equity makes the projection fall back to summing
+        # position_closed events — honest, and correct now that fills/closes
+        # are journaled reliably.
+        "last_equity": None,
         "cash": _s(b.get("total_cash") or 0),
         "buying_power": _s(buying_power),
         "status": "ACTIVE",