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.

backend/entrypoint.sh

@@ -11,7 +11,7 @@ echo "Starting Siftlode API..."
 # on the next request -> 502. Non-idempotent POSTs (the player's progress/state writes,
 # fired every 5s) can't be auto-retried by the proxy, so those 502s reached the user.
 #
-# SECURITY: do NOT add --proxy-headers / --forwarded-allow-ips here. ratelimit.client_ip trusts
+# SECURITY: do NOT add --proxy-headers / --forwarded-allow-ips here. ratelimit.client_key trusts
 # X-Forwarded-For only when the TRUE socket peer is a configured proxy (TRUSTED_PROXY_IPS); those
 # flags would make uvicorn overwrite request.client from the forgeable XFF, defeating that check and
 # reopening the rate-limit bypass (SA3).