Leg 0.16.1: Three things that nagged every session, and the sharecard

Three things that nagged every session, and the sharecard.

• The terminal no longer claims "no board is reading claude usage" while
  one is. A 429 from the usage endpoint backs the board's poller off for up
  to ten minutes; the terminal read the stale record as "no board", said so,
  and started its own reads, one more request a minute at an endpoint already
  refusing. The poller now writes next_poll_at on every attempt, a refusal
  included, and the terminal stands down while that promise is not overdue
  (boardIsPolling, src/usage.mjs). A board that is genuinely gone still
  gets the old line and the old fallback.
• leg claude opens the board only when nobody is looking at it.
  /api/health reports viewers, the count of browser tabs on the event
  stream; a second terminal on a board that is already on screen adds no tab.
  A guarded board (share on) says no count and opens as before.
• A second claude login fits its row on the capacity strip. The name
  column was a fixed 7ch, and "claude/work" ran into its own track. The
  column is now as wide as the longest login on the strip, set by strip.js,
  so every row's track still starts flush.
• The sharecard reaches X. robots.txt said Disallow: /og to keep the
  card's source page out of the index, and that prefix also blocked
  /og.png, so X drew a bare link for four days. The line is /og$ with an
  explicit Allow: /og.png; the card's foot no longer overflows the frame
  and the image URL is ?v=5.