feat(letsplot): implement scatter-basic (#465)

## Summary
Implements `scatter-basic` for **letsplot** library.

**Parent Issue:** #207
**Sub-Issue:** #410
**Base Branch:** `plot/scatter-basic`
**Attempt:** 2/3

## Implementation
- `plots/letsplot/point/scatter-basic/default.py`

## Features
- Uses lets-plot ggplot2-style grammar of graphics
- Python Blue (#306998) color for data points
- Proper sizing (4800x2700px via scale=3)
- theme_minimal() with custom font sizes for readability
- Follows all quality criteria and style guide requirements

## Note
Previous PR #449 was closed due to transient GitHub Actions
infrastructure timeout (unrelated to code).

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

Author: claude[bot]

plots/letsplot/point/scatter-basic/default.py

@@ -5,7 +5,7 @@
 
 import numpy as np
 import pandas as pd
-from lets_plot import LetsPlot, aes, element_text, geom_point, ggplot, ggsave, ggsize, labs, theme
+from lets_plot import LetsPlot, aes, element_text, geom_point, ggplot, ggsave, ggsize, labs, theme, theme_minimal
 
 
 LetsPlot.setup_html()
@@ -23,6 +23,7 @@
     + geom_point(color="#306998", size=4, alpha=0.7)
     + labs(x="X Value", y="Y Value", title="Basic Scatter Plot")
     + ggsize(1600, 900)
+    + theme_minimal()
     + theme(plot_title=element_text(size=20), axis_title=element_text(size=20), axis_text=element_text(size=16))
 )