feat(letsplot): implement line-basic (#460)

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

**Parent Issue:** #205
**Sub-Issue:** #456
**Base Branch:** `plot/line-basic`
**Attempt:** 1/3

## Implementation
- `plots/letsplot/line/line-basic/default.py`

## Details
- Uses `geom_line()` with `geom_point()` overlay for data point
visibility
- Python Blue (#306998) color for consistent styling
- Properly sized output (4800 × 2700 px via scale=3)
- Follows letsplot ggplot2-style grammar of graphics

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Markus Neusinger <2921697+MarkusNeusinger@users.noreply.github.com>

Author: 3 people

plots/letsplot/line/line-basic/default.py

@@ -28,12 +28,12 @@
 plot = (
     ggplot(data, aes(x="time", y="value"))
     + geom_line(color="#306998", size=2)
-    + geom_point(color="#306998", size=4)
+    + geom_point(color="#306998", size=4, alpha=0.8)
     + labs(x="Time", y="Value", title="Basic Line Plot")
     + theme_minimal()
     + theme(plot_title=element_text(size=20), axis_title=element_text(size=20), axis_text=element_text(size=16))
     + ggsize(1600, 900)
 )
 
-# Save - scale 3x to get 4800 x 2700 px
+# Save (scale 3x to get 4800 × 2700 px)
 ggsave(plot, "plot.png", path=".", scale=3)