Handle character width and offscreen rendering

[Shresht7]

Implement logic to account for character widths in rendering and movement calculations.

Introduce a private width field for entities and optimize boundary checks for offscreen conditions.

Adjust random symbol generation to respect column widths and improve trail clearing logic for right-to-left movement.

Without the unicode-width changes, horizontal/diagonal movements had these issues:

• Positioning: Entities were spaced by i (1 column) instead of i * width, so double-width chars overlapped each other
• Speed: rain() moved x by speed_x instead of speed_x * width, so horizontal flow was too slow for wide chars
• Off-screen detection: Checked e.x >= columns instead of e.x + width >= columns, so entities hung off the right edge
• Trail clearing: Used a single space instead of " ".repeat(width), leaving ghost characters
• Background population (the --leave-trail option): Wrote a char to every column, so double-width chars stomped on each other
• is_offscreen took &mut self unnecessarily