fix(keys): epic-end review round 2 — focus comes back to the card, the right rail and the reason

Whole-epic deep-review of E11 (18 raised, 12 kept; B and G refuted, four latent test-hardening notes dropped),
plus the admin refusal copy gap round 1 surfaced.

- A context menu opened from a card the keyboard is on gives the focus back to the card. floating-ui returns
  focus through the opener's first tabbable descendant even when handed a ref, so for a non-tabbable opener the
  surface restores it itself (measured in Chrome: j, Shift+F10, Escape left it on the thumbnail link).
- Back finds a named list (a rail) by its name: a rail that appeared above it after the cache expired sent the
  focus into the wrong rail. An unnamed list is matched only when the page has the same number of lists.
- The slot kept for a removed item is forgotten once focus moves on to page content (search, a chip), so j after
  new results starts at the first visible item; a menu or dialog opened from the item keeps it.
- The list keys reach the cheat sheet only while a list has an item.
- A j-focused card root is a named group while it holds the focus.
- The channel page's Block and Hide toggles keep the focus while their request runs.
- A split export that fails part-way says how many clips were added and deselects them, so a retry adds only
  the rest.
- Config save, the one-click share link and "log out other sessions" report failures with their reason.
- VirtualGrid uses the one scroll-parent walk listNav measures against.
- Admin refusals (activate, resend, invite, set-password link) say why in HU and EN instead of "try again";
  the email-off special cases collapse into that one reason. The contract reads the codes from localizeDetail.
  HU copy points to Konfiguráció → E-mail / SMTP; HU "unreachable" matches the EN change of round 1.

frontend/src/components/ChannelPage.tsx

@@ -479,11 +479,13 @@ export default function ChannelPage({
               )}
               {!me.is_demo && (
                 <button
-                  onClick={() => block.mutate()}
-                  disabled={block.isPending}
+                  // Busy blocks the click, not the focus: a focused button that turns natively
+                  // `disabled` drops the keyboard to <body> (the rule and its measurement: ui/Button).
+                  onClick={() => !block.isPending && block.mutate()}
+                  aria-disabled={block.isPending || undefined}
                   title={ch?.blocked ? t("channel.unblock") : t("channel.block")}
                   aria-label={ch?.blocked ? t("channel.unblock") : t("channel.block")}
-                  className={`inline-flex items-center justify-center w-9 h-9 rounded-lg border disabled:opacity-50 transition ${
+                  className={`inline-flex items-center justify-center w-9 h-9 rounded-lg border aria-disabled:opacity-50 transition ${
                     ch?.blocked
                       ? "border-danger text-danger"
                       : "border-border text-muted hover:text-danger hover:border-danger"
@@ -497,11 +499,11 @@ export default function ChannelPage({
                   from the feed. EyeOff = currently hidden (click to unhide). */}
               {ch?.subscribed && !me.is_demo && (
                 <button
-                  onClick={() => toggleHidden.mutate()}
-                  disabled={toggleHidden.isPending}
+                  onClick={() => !toggleHidden.isPending && toggleHidden.mutate()}
+                  aria-disabled={toggleHidden.isPending || undefined}
                   title={ch.hidden ? t("channel.hiddenHint") : t("channel.hideHint")}
                   aria-label={ch.hidden ? t("channel.unhide") : t("channel.hide")}
-                  className={`inline-flex items-center justify-center w-9 h-9 rounded-lg border disabled:opacity-50 transition ${
+                  className={`inline-flex items-center justify-center w-9 h-9 rounded-lg border aria-disabled:opacity-50 transition ${
                     ch.hidden
                       ? "border-accent text-accent"
                       : "border-border text-muted hover:text-fg hover:border-fg/40"