Rework Tasks tab: Chains section, checklist modal, project visibility

The flat Tasks-tab atom list was silently dumping every chain step
(locked and unlocked) and dormant bucket-pool items in as ordinary
undated cards, with no chain/project context and no protection against
completing a locked step out of order.

- CompleteNativeTask now rejects completing a locked chain task
  (ErrChainTaskLocked), mapped to 400 in both the widget and web
  complete-atom handlers.
- Chain tasks and dormant bucket items are excluded from the flat atom
  list; a new "Chains" section shows one card per active/paused chain
  with the current step and N/M progress.
- New chain checklist modal (GET /chains/{id}) lists every position in
  order with pause/resume/abandon -- the web view originally deferred
  as Android-only.
- Fixed a real bug this surfaced: resuming a paused chain only flipped
  the status flag, never unlocking the deferred successor, so a chain
  paused right after a completion stayed stuck forever. SetChainStatus
  now catches up the deferred advancement on resume, idempotently.
- Atom cards gained a project-name chip for general visibility.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ7ikw2ukGJFTHE3bJS7zL

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