feat(highcharts): implement area-basic (#505)

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

**Parent Issue:** #201
**Sub-Issue:** #473
**Base Branch:** `plot/area-basic`
**Attempt:** 1/3

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

## Features
- Basic area chart showing monthly sales trend
- Python Blue (#306998) color scheme
- Filled area with 50% opacity
- Visible markers on data points
- Subtle grid lines (10% opacity)
- Full x-axis labels (Jan-Dec) and y-axis label (Sales $)
- 4800x2700px output via Selenium screenshot

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

Author: claude[bot]

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

@@ -38,9 +38,10 @@
 chart.options.x_axis = {
     "title": {"text": "Month", "style": {"fontSize": "40px"}},
     "categories": ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"],
-    "labels": {"style": {"fontSize": "32px"}},
+    "labels": {"style": {"fontSize": "32px"}, "enabled": True},
     "gridLineWidth": 1,
     "gridLineColor": "rgba(0, 0, 0, 0.1)",
+    "tickmarkPlacement": "on",
 }
 
 chart.options.y_axis = {
@@ -93,12 +94,16 @@
 chrome_options.add_argument("--no-sandbox")
 chrome_options.add_argument("--disable-dev-shm-usage")
 chrome_options.add_argument("--disable-gpu")
-chrome_options.add_argument("--window-size=4800,2700")
+chrome_options.add_argument("--window-size=4800,2800")
 
 driver = webdriver.Chrome(options=chrome_options)
+driver.set_window_size(4800, 2800)
 driver.get(f"file://{temp_path}")
 time.sleep(5)
-driver.save_screenshot("plot.png")
+
+# Get the container element and screenshot it directly
+container = driver.find_element("id", "container")
+container.screenshot("plot.png")
 driver.quit()
 
 Path(temp_path).unlink()