feat: Big Number with YoY & MoM chart plugin

## Feature request

Add a new ECharts chart type "Big Number with YoY & MoM" that renders a KPI card with an optional title, a big number, and MoM/YoY comparison rows (reference: "Big Number with Trendline").

### Highlights
- Two mutually exclusive comparison sources per slot: time shift (percentage change) or comparison value metric (column / custom SQL rendered as the computed value with up/down direction arrows)
- Time column (X axis) + time grain; adhoc SQL time columns supported
- Open time ranges use point-to-point comparison (missing points render as "—"), no enclosed time range required
- 5-tier ratio font sizes, configurable positions/gaps (blank = 0), auto-spacing against overlap
- Big number number/currency format independent from MoM/YoY formats
- Chinese translations and light/dark thumbnails

Implemented in PR #44138.

Start with the implemented PR #44138 and compare it with the issue requirements and the existing “Big Number with Trendline” chart. Verify that the comparison sources, time ranges, formatting, spacing, translations, and light/dark thumbnails are covered before considering the feature complete.

Written by the indexing model from the issue text.