ui(pool): pick an account's users from its own Edit

Each account row in the Account pool card has a pencil: tick the users
whose sessions run on it (one moves off any other account, unticking sends
one back to the balancer). PUT /api/accounts/:name/users saves the set in one
transaction, audits each change and switches a Claude login into each added
user's home at once. The card-level switch button and pinned list are gone.

docs/SERVER-MODE.md

@@ -425,9 +425,11 @@ How a new session picks its account (`server/lib/claude-pool.js`):
 - A token more than a year old is skipped.
 - A resumed session picks an account again, like a new one.
 - **Switching a user to one account.** An admin can take one user off the
-  ranking: **Switch a user's account** in the Account pool card (or
-  `agenthub accounts pin <account> <user> [--tool codex]`, `unpin <user>` for
-  automatic again; their own password again, audited). Every session in a
+  ranking: the pencil (**Edit**) on that account's row in the Account pool
+  card — tick the users who run on it, untick to send one back to automatic (or
+  `agenthub accounts pin <account> <user> [--tool codex]`, `unpin <user>`; their
+  own password again, each change audited). A user ticked on one account
+  moves off any other. Every session in a
   workspace that user OWNS then runs on that account, even when it is full;
   other users are not affected, and the account is not reserved for them.
   - **A Claude login switches at once, without a restart.** The server first