BACK-633 - Scroll the task detail pane with Shift+J/K without leaving the list

[kossakovsky]

Summary

In the task-list view the detail pane can only be scrolled after moving focus into it (right/l or Enter). This binds Shift+J / Shift+K as screen-level shortcuts that scroll the detail pane body from either pane, so the list keeps focus while long descriptions are read.

• inert while the filter bar is focused, a popup or modal is open, or there is no detail pane
• plain j/k list navigation is untouched (uppercase-only bindings, following the existing ['e','E','S-e'] idiom)
• the help popup documents the shortcut

Rebased onto current main

Rebased from the July branch point onto current main. Two notes on how this interacts with what shipped since:

• BACK-584 (BACK-584 - Keep vim keys inside the list at navigation boundaries #866) reshaped boundary navigation after this PR was opened. The original description also claimed "never triggers the boundary handoff into the search field" — that clause is now satisfied by BACK-584 - Keep vim keys inside the list at navigation boundaries #866 independently, since k at the top of the detail pane already stays put. The remaining scope of this PR is only the scroll-without-leaving-the-list behaviour. The new helper composes with the current shouldMoveFromDetailBoundaryToSearch(scrollOffset, key) signature and adds no interaction with it; the existing navigation suites pass unchanged.
• Task ID reallocated. This was originally filed as BACK-537, but main allocated that ID to "Make checklist edits and serialization deterministic" on 2026-07-11. To avoid introducing a duplicate task ID, the task was recreated through the CLI allocator as BACK-633 and the old file dropped. The branch name still says back-537 because renaming it would close this PR.

Related Issue or Task

Closes #769

Backlog task: BACK-633

Task Checklist

• I have created a corresponding task in backlog/tasks/
• The task has clear acceptance criteria
• I have added an implementation plan to the task
• All acceptance criteria in the task are marked as completed

Testing

• bunx tsc --noEmit — clean
• bun run check . — clean (374 files)
• bun run test — 2250 passed / 6 skipped / 1 failed. The single failure is src/test/tui-window-title.test.ts, which fails identically on unmodified main in my environment (I run tests inside tmux, which rewrites the terminal escape sequences that test asserts on), so it is pre-existing and unrelated to this change.
• src/test/task-viewer-detail-scroll.test.ts covers all six branches of the new helper.
• Real-PTY check (tmux, 200x50, temp project with a 60-line description), since viewTaskEnhanced is not exercised by the test suite: five Shift+J scrolled the detail body without changing the selected task, three Shift+K scrolled back, and a subsequent plain j moved the selection — confirming focus never left the list. With the search field focused, J was inserted as a literal character and the detail pane did not scroll.

Happy to add a key-event test in the style of src/test/tui-vim-boundary-navigation.test.ts if you'd prefer that behaviour locked down in CI rather than verified by hand.