feat(ui): multi-column help overlay with scrolling

[i-doll]

Summary

• Restructure help popup to use 1–3 columns based on terminal width (thresholds at 82/126 inner width), with greedy shortest-column section distribution
• Add j/k/J/K/Up/Down/PageUp/PageDown scrolling in help mode, with ▲/▼ indicators when content overflows
• Popup width and height adapt dynamically to column count and terminal size

Test plan

• cargo clippy — no warnings
• cargo test — all 553 tests pass (including new test_help_scroll and test_help_mode_scroll_keys)
• Manual: open help on wide terminal (≥88 cols) → 2 columns
• Manual: open help on very wide terminal (≥128 cols) → 3 columns
• Manual: open help on narrow terminal (<88 cols) → single column with j/k scrolling
• Manual: resize terminal while help is open → columns reflow