BACK-466 - Add Vim jump and half-page motions to TUI board columns

[alessandro-rizzo]

Summary

Adds familiar Vim vertical-navigation motions to the focused column of the terminal Kanban board (backlog board), extending the existing h/j/k/l bindings:

Key	Action
gg	Jump to the first task in the column (two g presses within 400ms; a lone g is a no-op)
G	Jump to the last task
Ctrl+d	Move selection down half the column's visible height
Ctrl+u	Move selection up half the column's visible height

All four motions clamp inside the column and never hand focus to the search box, so the existing j/k boundary→search behavior is unchanged. They also work in move mode, repositioning the move target (top/bottom/half-page) within the valid range.

Full-page Ctrl+f/Ctrl+b is intentionally out of scope to avoid clashing with the existing Ctrl+f search shortcut.

Implementation

• src/ui/task-viewer-with-search.ts — new pure, exported resolveVimMotionIndex(motion, currentIndex, totalItems, visibleHeight) (+ VimVerticalMotion type) that does all clamped index math, mirroring the existing shouldMoveFromListBoundaryToSearch / resolveSearchExitTargetIndex helpers.
• src/ui/board.ts — four thin screen.key handlers + a shared applyVimMotion closure that reads the list's rendered height (falling back to task count) and routes through selectColumnRow (navigation) or moveOp.targetIndex (move mode).
• src/ui/components/help-popup.ts — added gg/G and C-d/u rows to the board shortcuts.
• src/test/board-vim-motion.test.ts — unit tests for the helper (top/bottom, half-page clamping both ends, empty list, odd-height rounding, tiny-height minimum step).

Testing

• bunx tsc --noEmit — passes
• bun run check . (changed files) — passes
• bun test — 1250 pass / 0 fail

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