Add currencies tab to markets app

[Copilot]

Markets page only covered crypto, futures, and commodities. Adds a fourth Currencies tab tracking major fiat currencies (EUR, GBP, JPY, CAD, AUD, CHF, CNY, INR) priced in USD with 24h change.

Changes

• New category: CategoryCurrencies = "currencies" constant; tab rendered in generateMarketsPage; category validated in Handler
• Data source: Yahoo Finance forex quotes via quote.Get (e.g. EURUSD=X) — same library already used for futures, providing both spot price and RegularMarketChangePercent
• Chart links: Each currency links to its Yahoo Finance forex chart (/chart/EURUSD%3DX, etc.)
• forexSymbols map: Maps currency codes → Yahoo Finance ticker symbols
• getAssetsForCategory: Extended to return currencyAssets for the new category

var forexSymbols = map[string]string{
    "EUR": "EURUSD=X",
    "GBP": "GBPUSD=X",
    "JPY": "JPYUSD=X",
    // ...
}

Prices are fetched alongside futures in fetchPrices, using the same IIFE/recover pattern. Currency data is persisted and cached identically to other asset classes.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.