feat(altair): implement area-basic (#432)

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

**Parent Issue:** #201
**Sub-Issue:** #395
**Base Branch:** `plot/area-basic`
**Attempt:** 1/3

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

## Changes
- Uses `mark_area()` with inline `line` parameter for cleaner code
- Extended data from 6 to 12 months for richer visualization
- Added `configure_view(strokeWidth=0)` to remove chart border
- Configured proper font sizes for title, axis labels, and tick labels
- Subtle grid with `gridOpacity=0.3`
- Outputs 4800×2700px PNG (1600×900 × scale_factor=3.0)

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

Author: claude[bot]

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

@@ -8,25 +8,25 @@
 
 
 # Data
-data = pd.DataFrame({"month": [1, 2, 3, 4, 5, 6], "sales": [100, 150, 130, 180, 200, 220]})
+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],
+    }
+)
 
 # Create chart with area and line
-area = (
+chart = (
     alt.Chart(data)
-    .mark_area(opacity=0.5, color="#306998")
+    .mark_area(opacity=0.5, color="#306998", line={"color": "#306998", "strokeWidth": 2})
     .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))
+    .properties(width=1600, height=900, title=alt.TitleParams(text="Basic Area Chart", fontSize=20))
+    .configure_view(strokeWidth=0)
     .configure_axis(grid=True, gridOpacity=0.3)
 )
 
-# Save
+# Save (1600 × 900 × 3 = 4800 × 2700)
 chart.save("plot.png", scale_factor=3.0)