Move threads between sidebar sections from row actions

[brsbl]

Human comments

What was wrong

• Custom-organized sidebar threads could be moved by drag and drop, but their row actions offered no destination picker.

What changed

• Adds Move to section with MoveTo to active root-thread actions in Custom view: desktop dropdown/right-click submenus and a compact long-press step with Back.
• Follows saved sidebar order, including Threads, and omits Pinned. The current destination is disabled for unpinned threads. Existing mutations preserve move persistence; moving a pinned thread unpins it, and choosing its stored section only unpins it.
• Integrates with the shared sidebar header controls from Unify thread-list header actions and nested view controls #3402. The focused diff remains 12 files; main's bundle limits are unchanged.

How you verified

• User QA passed on this candidate. Squash commit 839ceef97c6a1dd8384703127dc655f8ac1aab16 has the identical source tree; post-merge CI passed all 13 jobs.
• Remote CI for d334e6953963fd5891a1fdb7eb75f020cd604d0f: 14 passed, 2 skipped, including checks, app/server/package/integration tests, and macOS/Ubuntu package smoke. Optional Node compatibility and native iOS jobs were skipped. The app fits unchanged bundle limits: 1681.7 KB raw / 410.9 KB brotli against 1683.2 KB / 419.0 KB.
• Menu and mutation tests cover desktop/context parity, eligibility, compact Back/reset, destination no-op, section moves, and both pinned transitions. Existing optimistic-update and serialized-unpin coverage remains.
• Final-head Chrome for Testing 153.0.8010.36: shared header By project → Custom eligibility; desktop dropdown and right-click moves to a section and Threads; hard-reload persistence and disabled current destination; compact touch long-press, Back, close/reopen reset, and saved moves. No captured runtime errors or compact app-root inert/aria-hidden changes. All six synthetic threads and sidebar settings were restored. The unchanged pinned transitions were exercised before this rebase and retain passing remote mutation coverage.
• Native iOS is unverified: the local Xcode/CoreSimulator setup is incompatible and cannot resolve simctl; remote iOS coverage was skipped. Phone-width Chrome does not establish native iOS or Safari behavior.

Surface	Before — merge base de9361268088	After — head d334e6953963
Desktop web · 1440 × 900
Phone web · 390 × 844

• Both revisions use scripts/bb-dev-app current under Node 22.19.0, sequentially in the same managed worktree. Captures match the synthetic Release notes thread in Follow-ups, /threads/thr_move_release, Custom organization, light theme, scale 1, and viewport per row. Images were inspected directly.

BB-Thread-ID: thr_ccffp4w2p2

AGENT GENERATED