feat: tidy up plan decline + sort/paginate tasks page

[pufit]

Two small UX improvements landing on one branch.

1. Plan decline moves task to done

Declining a plan now moves the related task to the done/ directory with a note. Previously the task was left in pending status while the plan transitioned to declined — leaving an orphan task that had to be closed manually.

• UI: the Decline button expands an optional feedback textarea mirroring the existing Revise flow. A Confirm Decline button submits the action; the feedback is optional.
• Backend: both PATCH /api/plans/{id} and the plan_decline MCP tool route through task_done so file movement and DB status stay consistent across surfaces.
• Note text:
  • With feedback → Plan {id} declined — {feedback}
  • Without feedback → Related plan {id} was closed without a specified reason

2. Tasks page sort + pagination

• Three sort orders: Deadline (default), Last update, Created.
• 50-per-page pagination with prev/next controls and a Showing X–Y of N indicator.
• Sort/page reset to defaults when the filter/search/status changes.
• list_tasks gains sort and offset kwargs; new count_tasks helper returns the total ignoring pagination.
• A stable secondary key (id DESC) is appended to every ORDER BY so equal timestamps don't shuffle between pages.
• Each task card now shows a compact updated 2h ago label — makes the Last update sort visually meaningful.
• Search is not paginated — it returns the top 100 BM25-ranked hits and the pagination row is hidden, since slicing relevance results across pages fights the ranking.

Verification

• pytest tests/ -v → 542 passed (added one test covering sort orders, pagination slicing, and count_tasks).
• npm run build clean.

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