Consolidate two inconsistent codepoint_width implementations from
Screen.cpp and EventEditor.cpp into nocal/tui/unicode.hpp. The unified
table covers all CJK, Hangul, emoji, symbol, and combining mark ranges.
display_width and fit_text handle grapheme clusters (ZWJ sequences,
skin-tone/hair modifiers, regional indicator pairs, variation selectors)
so calendar text with emoji or East Asian characters renders correctly.
