🎨 Palette: Add context tooltips & fix dashboard crash

[google-labs-jules]

💡 What: Added help tooltips to "Transaction Cost", "High Volatility Quantile", and "Allow Short Selling" inputs. Also added matplotlib to requirements.txt.
🎯 Why:

1. Financial terms like "bps" and quantiles are ambiguous to many users. Tooltips provide concrete examples (e.g., "10 bps = 0.10%") to improve accessibility and confidence.
2. The "Backtest Engine" tab was crashing with an ImportError because background_gradient styling relies on matplotlib, which was missing from the environment.
   📸 Before/After: Added explanation for jargon; Fixed a visible crash in the dashboard.
   ♿ Accessibility: Users with screen readers or less domain knowledge can now understand the input parameters without external searching.

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PR created automatically by Jules for task 925698468371400485 started by @aarjava

[google-labs-jules]

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