feat(letsplot): implement line-basic (#450)

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

**Parent Issue:** #205
**Sub-Issue:** #334
**Base Branch:** `plot/line-basic`
**Attempt:** 1/3

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

## Changes
- Clean KISS implementation following style guide
- Uses Python Blue color (#306998) for line and points
- Proper 4800x2700 px output via ggsize(1600, 900) with scale=3
- Minimal theme with proper typography sizing

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

Author: claude[bot]

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

@@ -7,7 +7,6 @@
 from lets_plot import (
     LetsPlot,
     aes,
-    element_line,
     element_text,
     geom_line,
     geom_point,
@@ -28,17 +27,11 @@
 # Plot
 plot = (
     ggplot(data, aes(x="time", y="value"))
-    + geom_line(color="#306998", size=2, alpha=1.0)
-    + geom_point(color="#306998", size=4, alpha=1.0)
+    + geom_line(color="#306998", size=2)
+    + geom_point(color="#306998", size=4)
     + labs(x="Time", y="Value", title="Basic Line Plot")
     + theme_minimal()
-    + theme(
-        plot_title=element_text(size=20, face="bold"),
-        axis_title=element_text(size=20),
-        axis_text=element_text(size=16),
-        panel_grid_major=element_line(color="#CCCCCC", size=0.5),
-        panel_grid_minor=element_line(color="#EEEEEE", size=0.3),
-    )
+    + theme(plot_title=element_text(size=20), axis_title=element_text(size=20), axis_text=element_text(size=16))
     + ggsize(1600, 900)
 )