fix(assets): render banner SVG as <rect> primitives to kill horizontal gaps

The <text>-based banner had thin horizontal lines between rows because
block glyphs (▀/▄) aren't cell-tiled in SVG text flow — font fallback
on GitHub introduced line-height gaps that showed through the half-block
characters.

Replace with 120 <rect> cells laid out contiguously (12×26 per cell,
rows at y=20/46/72). No font dependency, no gaps between rows. Rows
keep the blue gradient (#7bb8ff / #4a9eff / #1b45a8), and the dotted
shade chars (░) render at opacity 0.22 for subtle texture.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: senna-lang