Time Slider UI

## Description  
Introduce a time selection UI similar to Design Explorer / Histograph.

## Tasks  
- [ ] Add slider container in index.md  
- [ ] Integrate noUiSlider  
- [ ] Initialize slider with min/max year  
- [ ] Emit selected [startYear, endYear]  

## Suggested UI  

```html
<div id="yearSlider"></div>
<div id="yearRangeLabel"></div>