perf: downsample overview pnl chart points

[christianarriaga1234-coder]

Summary

Refs #2.

This PR reduces the render load of the Overview PnL chart when accounts accumulate large trade histories. It keeps the cumulative PnL series accurate, then applies Largest-Triangle-Three-Buckets downsampling before handing data to Recharts so the chart renders a bounded number of representative points.

Type

• Performance improvement

Changes

• Added a 600-point render cap for the Overview PnL chart.
• Added LTTB downsampling to preserve the first point, final cumulative PnL, and representative shape/extremes.
• Kept the existing chart API and no-data behavior unchanged.

Performance validation

• Synthetic 50,000-point PnL series downsampled to 600 rendered points.
• Point reduction: ~83.3x fewer chart points for Recharts to lay out/render.
• Local micro-benchmark for the downsampling pass: 8.459ms on 50,000 points via Node 26 bundled runtime.

Testing

• Ran a local Node micro-benchmark of the downsampling helper logic.
• Verified the first and final cumulative PnL values are preserved in the sampled output.
• Could not run Next lint/build locally because this clone does not include node_modules and dependencies were not installed.

Checklist

• No secrets, API keys, or private keys committed
• No backend schema changes
• Single focused UI performance change

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