feat: Allow custom range and interval in market widget

[suffer-sami]

Closes #974

Summary

This PR adds support for customizing the range and interval parameters used when fetching chart data for the market widget.

Previously, the API request hardcoded these values:

https://query1.finance.yahoo.com/v8/finance/chart/%s?range=1mo&interval=1d

This now allows us to use custom range and also to fetch data for symbols that require different parameters as mentioned in issue #974 (e.g. VNINDEX only returns data with a 4h interval and a short range).

Changes

• Added range and interval fields to the marketRequest struct
• Defaults to range: 1mo / interval: 1d when not specified (fully backwards compatible)
• Returns a clear config error for invalid values at startup
• Updated docs/configuration.md with the new properties and valid values

Example

- type: markets
 markets:
   - symbol: SPY
     name: S&P 500
   - symbol: BTC-USD
     name: Bitcoin
     chart-link: https://www.tradingview.com/chart/?symbol=INDEX:BTCUSD
   - symbol: NVDA
     name: NVIDIA
   - symbol: AAPL
     symbol-link: https://www.google.com/search?tbm=nws&q=apple
     name: Apple
   - symbol: ^VNINDEX.VN
     name: VNINDEX (4h)
     interval: 4h
     range: 1mo

Screenshot