feat(pygal): implement area-basic (#447)

## Summary
Implements `area-basic` for **pygal** library.

**Parent Issue:** #201
**Sub-Issue:** #427
**Base Branch:** `plot/area-basic`
**Attempt:** 4/3

## Implementation
- `plots/pygal/area/area-basic/default.py`

## Features
- Creates a filled area chart using pygal's Line chart with `fill=True`
- Uses PyPlots color palette (#306998 - Python Blue)
- 4800x2700 pixel output (16:9 aspect ratio)
- Custom style with appropriate font sizes for readability
- Clean axes labels (Month, Sales ($))
- Subtle grid on Y-axis

## Fix for CI Issue
Previous attempts failed because:
1. The base branch has a duplicate file at
`plots/pygal/line/area-basic/default.py`
2. Deleting this duplicate caused CI to try to test the deleted file
(which doesn't exist)

This attempt keeps only the correct `area/` implementation in the diff
without touching the `line/` duplicate. The duplicate should be cleaned
up separately.

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Markus Neusinger <2921697+MarkusNeusinger@users.noreply.github.com>

Author: 3 people

plots/pygal/area/area-basic/default.py

@@ -7,7 +7,7 @@
 from pygal.style import Style
 
 
-# Data
+# Data - monthly sales example from spec
 months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun"]
 sales = [100, 150, 130, 180, 200, 220]
 
@@ -29,7 +29,7 @@
     opacity_hover=0.7,
 )
 
-# Create chart
+# Create area chart (Line chart with fill=True)
 chart = pygal.Line(
     width=4800,
     height=2700,