refactor(ui): simplify truncPad via runewidth Truncate/FillRight

[peczenyj]

What

Replace truncPad's hand-rolled width-aware truncate-and-pad loop with runewidth's built-in Truncate/FillRight helpers:

func truncPad(s string, w int) string {
	if w <= 0 {
		return ""
	}
	s = strings.ReplaceAll(s, "\t", "    ")
	return runewidth.FillRight(runewidth.Truncate(s, w, "…"), w)
}

go-runewidth is already a direct dependency, so this adds no modules while removing the manual width bookkeeping (ellipsis-cell reservation, wide-rune boundary handling, padding).

Why

This is the resolution of the side-by-side rendering library investigation: the lightest alternative to lipgloss / charmbracelet/x/cellbuf / muesli/reflow is no new library at all. Those only buy ANSI-aware truncation, which we don't need — color is applied after truncPad, so cells are always plain text when truncated.

Verification — output is byte-for-byte identical

• ✅ Side-by-side golden fixture unchanged
• ✅ TestTruncPadUnicode / TestTruncPadWideRunes (incl. 世界世→世界…)
• ✅ Differential check vs the original implementation: identical across 21 inputs × 28 widths (negatives, zero, CJK, tabs, emoji, accents, wide-rune-straddling)
• ✅ task ci green (202 tests, lint 0 issues)

Closes #93

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