[KodeStar]

Closes #137.

The admin book page gains Listen in player (beside Match, only with the web_player capability). A click opens a tab synchronously (so no pop-up block), mints a one-time token with the existing POST /auth/pair, and points the tab at the same-origin /web/connect?token=&sid=&uid=&book=&path= (never the payload's web_url, which may be another origin). If the call fails the tab closes and a toast says why; one token per click.

No Go change. Pairs with KodeStar/audiosilo-frontend (feat/listen-in-player), which reads book/path/uid: already signed in as that user it opens the book without an exchange (after a GET /me check, so a revoked token falls through to the exchange), else it pairs and ends on the book. /web ships with the server, so the two land together.

Verification

• Console tests: URL shape, opener cleared, one pair call however fast the clicks, tab closed on failure, hidden without the web player.
• Real browser (headless Chromium, player built from the frontend branch): first click = pair + exchange, lands on the book signed in, playback runs; second click = pair + GET /me, no exchange; after revoking the device = GET /me 401 then exchange, lands signed in; a bogus link behaves as any failed pairing link.

Known: a skipped exchange leaves that click's single-use token unspent until it expires (10 minutes). Pairing tokens are never pruned from tokens (pre-existing); worth a small retention job later.

Docs: KodeStar/audiosilo-docs (docs/listen-in-player).