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/lib/api/core.ts

@@ -6,7 +6,7 @@ import { getActiveToasts, notify, remove } from "../notifications";
 import i18n from "../../i18n";
 import { formatBytes } from "../format";
 import { reportError } from "../errorDialog";
-import { activeAccountId, ACTIVE_ACCOUNT_KEY, pinActiveAccount } from "../activeAccount";
+import { activeAccountId, pinActiveAccount } from "../activeAccount";
 
 /** The active UI language, as a bare code the server understands ("en"/"hu" — never "en-GB").
  *  Lives here so the request builders can stamp it themselves: a signal that has to be passed in by
@@ -263,13 +263,6 @@ export function beacon(url: string, body: Record<string, unknown>): void {
 }
 
 export const setActiveAccount = pinActiveAccount;
-export function clearActiveAccount(): void {
-  try {
-    sessionStorage.removeItem(ACTIVE_ACCOUNT_KEY);
-  } catch {
-    /* ignore */
-  }
-}
 
 export async function req<T = unknown>(
   url: string,