feat(altair): implement area-basic (#524)

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

**Parent Issue:** #514
**Sub-Issue:** #519
**Base Branch:** `plot/area-basic`
**Attempt:** 1/3

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

### Changes from initial template:
- Used spec example data with proper month names (Jan-Dec) as ordinal
values
- Adjusted opacity to 0.7 for better visual clarity
- Added `sort=None` to preserve month order
- Configured proper axis labels and font sizes per style guide

Closes #519

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

Author: claude[bot]

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

@@ -10,20 +10,20 @@
 # Data
 data = pd.DataFrame(
     {
-        "month": [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12],
-        "sales": [100, 150, 130, 180, 200, 220, 195, 240, 260, 230, 280, 310],
+        "month": ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"],
+        "sales": [120, 135, 148, 162, 175, 195, 210, 198, 185, 170, 158, 190],
     }
 )
 
 # Create chart with area and line
 chart = (
     alt.Chart(data)
-    .mark_area(opacity=0.5, color="#306998", line={"color": "#306998", "strokeWidth": 2})
+    .mark_area(opacity=0.7, color="#306998", line={"color": "#306998", "strokeWidth": 2})
     .encode(
-        x=alt.X("month:Q", title="Month", axis=alt.Axis(labelFontSize=16, titleFontSize=20)),
+        x=alt.X("month:O", title="Month", sort=None, axis=alt.Axis(labelFontSize=16, titleFontSize=20)),
         y=alt.Y("sales:Q", title="Sales", axis=alt.Axis(labelFontSize=16, titleFontSize=20)),
     )
-    .properties(width=1600, height=900, title=alt.TitleParams(text="Basic Area Chart", fontSize=20))
+    .properties(width=1600, height=900, title=alt.TitleParams(text="Monthly Sales", fontSize=20))
     .configure_view(strokeWidth=0)
     .configure_axis(grid=True, gridOpacity=0.3)
 )