Update get_account_balance to use sum(weight) for SATS

Replace sum(position) with sum(weight) for efficient SATS aggregation
from price notation. Also return fiat amount from sum(number).

This simplifies the parsing logic and provides consistent SATS totals
across all BQL-based balance methods.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

views_api.py

@@ -309,7 +309,8 @@ async def api_get_account_balance(account_id: str) -> dict:
     return {
         "account_id": account_id,
         "balance": balance_data["sats"],  # Balance in satoshis
-        "positions": balance_data["positions"]  # Full Beancount positions with cost basis
+        "fiat": float(balance_data.get("fiat", 0)),  # Fiat amount
+        "fiat_currency": balance_data.get("fiat_currency", "EUR")
     }