Answering the planner from the board returns to the board #702

Follow-up to #678 (#614), from its review: answering from the board's "the planner declined" line now returns to the board.

• The board's form carries back=board. issueCommentSubmit sends the person back to /board with "Answered on #n. The planner plans again." back is a fixed word, never a URL: any other value returns to the issue as before, so this adds no open redirect (tested).
• The path: the owner is paged and opens the board. The line shows the planner's words and a box; he types an answer and presses Answer. He lands back on the board with the line gone and the flash confirming it.
• Removes: the detour to the issue page after answering. Adds no screen or control.

Looks (taken locally, 1280 and 390 px):

• Before: the amber line under the header with the planner's words, the "Answer the planner" box and the Answer button. On the phone the words go full width above the box.
• After answering: the green flash "Answered on #1. The planner plans again." across the top, the line gone, the regions unchanged.

Tests: test/planner-declined.test.ts now answers through the board's own form (303 to /board, decline cleared everywhere). It adds a test that an unknown back returns to the issue. tsc is clean; planner-declined, board and issues tests pass (35).