fix: review round 4 — an account switch unloads as the old account, a share link's lockout costs five networks

Deep-review round 4, targeted at round 3's changes (f569b19c..e7f38396).

Accounts
- Switching, adding or signing out an account moved the tab's pin before
  the page unloaded, so the players' resume beacons fired on the way out
  under the NEW account: one account's playback position saved into the
  other. The pin is now handed to the next page (pinOnNextPage) and taken
  over at import; clearActiveAccount is gone.
- Switching to a suspended account was refused but left it in the
  switcher, refused on every click; a deactivated one was not refused at
  all, and made the default it signed every account in the browser out on
  the next request. Every account _session_user would refuse — gone,
  deactivated, suspended, revoked — is refused and dropped from the list.

Share page (/watch)
- An IPv6 client is limited by its /64 (client_ip becomes client_key, for
  every limiter): keyed on the full address, one subscriber could rotate
  through endless identities. The comments now state what the per-link
  ceiling costs: guessing from five networks can keep a link shut for its
  window, and guesses already running when it fills are still checked.
- The link is looked up before the limiter records the attempt, so made-up
  tokens no longer grow it by an entry each.

Forms
- Failing the same way twice is announced again: the sign-in, passphrase
  and setup errors are keyed on the attempt (a clear and a set in one
  handler left the alert node untouched).
- The install wizard's own validation showed the generic error — it threw
  plain objects its catch did not read. It sets the step's message.

Tests: a re-pin leaving the previous account's notices behind; the pin
hand-over; switching to an ended account; the /64 viewer; made-up tokens;
the repeated alerts. Each mutation-checked RED for its own reason.

frontend/src/components/Welcome.tsx

@@ -398,6 +398,9 @@ function AuthCard({ verify }: { verify: VerifyEntry }) {
   const [password, setPassword] = useState("");
   const [busy, setBusy] = useState(false);
   const [error, setError] = useState("");
+  // Keys the error: failing the same way twice is a new node, so a screen reader announces it again (a clear
+  // and a set in one handler commit together, leaving the node untouched).
+  const [attempt, setAttempt] = useState(0);
   // Whether to offer "Continue with Google" — hidden when the instance has no Google OAuth
   // configured. Optimistic (most instances have it); the rare Google-less instance just sees it
   // disappear once the config loads.
@@ -481,6 +484,7 @@ function AuthCard({ verify }: { verify: VerifyEntry }) {
 
   async function onSubmit(e: React.FormEvent) {
     e.preventDefault();
+    setAttempt((n) => n + 1);
     reset();
     const mail = email.trim().toLowerCase();
     if (
@@ -687,7 +691,7 @@ function AuthCard({ verify }: { verify: VerifyEntry }) {
               />
             )}
             {error && (
-              <p role="alert" className="text-xs text-danger">
+              <p key={attempt} role="alert" className="text-xs text-danger">
                 {error}
               </p>
             )}