feat(highcharts): implement timeseries-forecast-uncertainty (#3282)

## Implementation: `timeseries-forecast-uncertainty` - highcharts

Implements the **highcharts** version of
`timeseries-forecast-uncertainty`.

**File:**
`plots/timeseries-forecast-uncertainty/implementations/highcharts.py`

**Parent Issue:** #3188

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20796526213)*

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

plots/timeseries-forecast-uncertainty/metadata/highcharts.yaml

@@ -3,10 +3,10 @@
 
 library: highcharts
 specification_id: timeseries-forecast-uncertainty
-created: '2026-01-07T16:33:22Z'
-updated: '2026-01-07T16:33:22Z'
+created: '2026-01-07T21:14:47Z'
+updated: '2026-01-07T21:14:47Z'
 generated_by: claude-opus-4-5-20251101
-workflow_run: 20788396560
+workflow_run: 20796526213
 issue: 3188
 python_version: 3.13.11
 library_version: unknown