feat(highcharts): implement area-basic (#468)

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

**Parent Issue:** #201
**Sub-Issue:** #428
**Base Branch:** `plot/area-basic`
**Attempt:** 1/3

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

## Features
- Basic area chart with monthly sales data
- Python Blue (#306998) color scheme following style guide
- 4800x2700 px output size
- Proper axis labels with readable font sizes
- Subtle grid (0.1 opacity)
- Markers on data points
- 50% fill opacity for area

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

Author: claude[bot]

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

@@ -15,50 +15,55 @@
 from selenium.webdriver.chrome.options import Options
 
 
-# Data
-months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
-sales = [100, 150, 130, 180, 200, 220, 195, 240, 260, 230, 270, 310]
+# Data - Monthly sales example
+sales = [100, 150, 130, 180, 200, 220, 195, 240, 280, 310, 290, 350]
 
 # Create chart
 chart = Chart(container="container")
 chart.options = HighchartsOptions()
 
 # Chart configuration
-chart.options.chart = {"type": "area", "width": 4800, "height": 2700, "backgroundColor": "#ffffff"}
+chart.options.chart = {
+    "type": "area",
+    "width": 4800,
+    "height": 2700,
+    "backgroundColor": "#ffffff",
+    "style": {"fontFamily": "Arial, sans-serif"},
+}
 
 # Title
-chart.options.title = {"text": "Monthly Sales Overview", "style": {"fontSize": "48px"}}
+chart.options.title = {"text": "Monthly Sales Trend", "style": {"fontSize": "48px"}}
 
 # Axes
 chart.options.x_axis = {
-    "categories": months,
-    "title": {"text": "Month", "style": {"fontSize": "36px"}},
-    "labels": {"style": {"fontSize": "28px"}},
+    "title": {"text": "Month", "style": {"fontSize": "40px"}},
+    "categories": ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"],
+    "labels": {"style": {"fontSize": "32px"}},
+    "gridLineWidth": 1,
+    "gridLineColor": "rgba(0, 0, 0, 0.1)",
 }
 
 chart.options.y_axis = {
-    "title": {"text": "Sales ($)", "style": {"fontSize": "36px"}},
-    "labels": {"style": {"fontSize": "28px"}},
-    "gridLineColor": "#e0e0e0",
+    "title": {"text": "Sales ($)", "style": {"fontSize": "40px"}},
+    "labels": {"style": {"fontSize": "32px"}},
     "gridLineWidth": 1,
+    "gridLineColor": "rgba(0, 0, 0, 0.1)",
 }
 
-# Add series
+# Legend
+chart.options.legend = {"enabled": False}
+
+# Create area series
 series = AreaSeries()
 series.data = sales
 series.name = "Sales"
 series.color = "#306998"
 series.fill_opacity = 0.5
 series.line_width = 4
+series.marker = {"enabled": True, "radius": 6, "fillColor": "#306998", "lineWidth": 2, "lineColor": "#ffffff"}
 
 chart.add_series(series)
 
-# Legend
-chart.options.legend = {"enabled": False}
-
-# Credits
-chart.options.credits = {"enabled": False}
-
 # Download Highcharts JS
 highcharts_url = "https://code.highcharts.com/highcharts.js"
 with urllib.request.urlopen(highcharts_url, timeout=30) as response: