fix(tui): ASCII logo renders letter n as o (opeocode)

What is the issue?

In the TUI splash screen, the ASCII logo renders as opeocode instead of opencode.

The 3rd row of the letter n in packages/tui/src/logo.ts and packages/tui/src/util/presentation.ts is defined with ▀~~▀. Because ~ renders a top-half block ▀ with shadow styling, it draws a continuous horizontal line across the bottom of the glyph between the two stems. This closes off the bottom of the letter n, making it look identical to the closed box o glyph.

Expected behavior

The space between the two stems of n should remain open (▀ ▀) rather than having a bottom horizontal bar connecting them.