feat(matplotlib): implement area-basic (#526)

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

**Parent Issue:** #514
**Sub-Issue:** #515
**Base Branch:** `plot/area-basic`
**Attempt:** 1/3

## Implementation
- `plots/matplotlib/fill_between/area-basic/default.py`

The implementation uses `fill_between` to create a filled area chart
with:
- Monthly sales data as specified in the spec
- Python Blue (#306998) color from the style guide
- Proper axis labels and title matching spec name
- Y-axis starting from 0 for accurate area representation
- Grid with subtle styling

Closes #515

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

Author: claude[bot]

plots/matplotlib/fill_between/area-basic/default.py

@@ -30,7 +30,7 @@
 # Labels and styling
 ax.set_xlabel("Month", fontsize=20)
 ax.set_ylabel("Sales", fontsize=20)
-ax.set_title("Monthly Sales Volume", fontsize=20)
+ax.set_title("Basic Area Chart", fontsize=20)
 ax.tick_params(axis="both", labelsize=16)
 ax.grid(True, alpha=0.3, linestyle="--")