Fix: Correct Fyers holdings data mapping

Resolved issues where:

• Total Investment Value was showing ₹0
• Total PnL Percentage was always 0%
• Avg Price was not displayed in holdings table

Updated fyers order_data mapping to correctly transform holdings data.

Now the holdings page correctly shows:

• Average Price
• Total Investment Value
• Accurate PnL %

Tested on local OpenAlgo dashboard.

Summary by cubic

Fixes Fyers holdings mapping so Avg Price, LTP, Total Investment Value, and PnL% display correctly in the dashboard. Maps costPrice to average_price, sets product from holdingType, rounds to 2 decimals, and computes pnlpercent with a divide-by-zero guard.

^{Written for commit fe407f9. Summary will update on new commits.}