curses: render graphs with Braille cells

[crgr]

Summary

This improves the curses graph rendering by using Unicode Braille cells for higher-resolution RX/TX graphs.

The previous graph renderer used one character per graph cell, which made curves and spikes look blocky. Braille cells provide multiple vertical sub-points per terminal cell, making the graph smoother while keeping the same overall graph dimensions.

Details

• Adds an optional GRAPH_CFG_BRAILLE graph mode.
• Keeps the existing classic single-character graph renderer available.
• Enables Braille rendering by default for curses output.
• Adds braille and nobraille curses output options.
• Initializes LC_CTYPE so ncurses can render UTF-8 Braille characters.
• Uses default/list background colors for Braille graph cells to avoid black blocks around the plot area.
• Leaves ASCII output unchanged.