Vest: the relayed login binds itself to an unset Vest firm

A Vest firm's credential is the wallet of the login it trades. It no longer
has to be typed or clicked in: when the extension relays a Vest tab's
session and a Vest firm here has no credential, that wallet is saved as the
credential (lib/vest-wallet.ts), the client starts, and the backup is
pushed.

- A saved wallet is never replaced. When the tab is signed in to another
  login, the firm now says which one and how to switch: sign in as the
  saved wallet, or remove the credential.
- With two logins relayed at once nothing binds itself; the one-click form
  decides.
- Extension 0.17.1 re-posts the session every minute even when it hasn't
  changed. The dashboard holds it in memory only, so a restart there was
  unauthenticated until the tab's next renewal (up to ~15 min); now it is
  back within a minute, and a firm created later binds within a minute too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -223,8 +223,12 @@ instance cannot log in by itself. Instead:
 2. The extension relays the tab's short-lived access token to this dashboard
    (`POST /api/vest/session`, held in memory only) every time the Vest app
    renews it. If no Vest tab is open, the extension opens one in the background.
-3. On the dashboard, the Vest firm's credential is the **wallet address** of
-   that login. The credential form offers the relayed wallet with one click.
+3. The Vest firm's credential on this dashboard is the **wallet address** of
+   that login, and it binds itself: the first relayed session fills it in when
+   the firm has none. A saved wallet is never replaced. If the tab is signed in
+   to a different login, the firm says so; remove its credential to switch.
+   With two logins relayed at once, nothing binds by itself, and the credential
+   form offers each relayed wallet with one click.
 
 With no live session for the wallet, the client sits unauthenticated and says
 so; it never tries to log in.