feat(altair): implement area-basic (#375)

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

**Parent Issue:** #201
**Sub-Issue:** #219
**Base Branch:** `plot/area-basic`
**Attempt:** 1/3

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

## Details
- Uses `mark_area()` with 0.5 opacity and Python Blue color (#306998)
- Overlays `mark_line()` for visible line on top of area
- Configures axis with subtle grid (opacity 0.3)
- 1600x900 base size with 3x scale factor for 4800x2700 output
- Font sizes: title 20pt, axis labels 20pt, tick labels 16pt

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

Author: claude[bot]

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

@@ -0,0 +1,32 @@
+"""
+area-basic: Basic Area Chart
+Library: altair
+"""
+
+import altair as alt
+import pandas as pd
+
+
+# Data
+data = pd.DataFrame({"month": [1, 2, 3, 4, 5, 6], "sales": [100, 150, 130, 180, 200, 220]})
+
+# Create chart with area and line
+area = (
+    alt.Chart(data)
+    .mark_area(opacity=0.5, color="#306998")
+    .encode(
+        x=alt.X("month:Q", title="Month", axis=alt.Axis(labelFontSize=16, titleFontSize=20)),
+        y=alt.Y("sales:Q", title="Sales", axis=alt.Axis(labelFontSize=16, titleFontSize=20)),
+    )
+)
+
+line = alt.Chart(data).mark_line(color="#306998", strokeWidth=2).encode(x="month:Q", y="sales:Q")
+
+chart = (
+    (area + line)
+    .properties(width=1600, height=900, title=alt.Title("Basic Area Chart", fontSize=20))
+    .configure_axis(grid=True, gridOpacity=0.3)
+)
+
+# Save
+chart.save("plot.png", scale_factor=3.0)