fix(ui): no button drops the keyboard focus when its own action disables it

A focused button that turns natively disabled loses the focus to <body> in
Chrome. The app disables the very button that was pressed on ~60 sites — a
mutation pending, a pager at its last page, undo with nothing left, a player
at the end of its queue, a form gone clean — so a keyboard user was thrown
out of the page on every such press.

One primitive instead of a fix per site: ui/FocusSafeButton renders a
focused-and-disabled (or busy) button as aria-disabled with a click guard,
and applies the native attribute again once the focus is elsewhere. It asks
the document at render time as well as listening for focus events, because
a window without focus fires none (measured). ui/Button and ui/form's Switch
are built on it; 57 hand-rolled buttons swapped their tag for it keeping
their classes, and three hand-copies of the rule (the channel page's Block
and Hide, ChannelDiscovery's Subscribe) now pass `busy`. The soft state's
dim look is one rule in index.css.

`local/focus-safe-disabled` refuses a new raw <button disabled>, pinned by
focusSafe.contract.test.ts. lib/plexDetailUi moved to components/: it is
components, and lib may not import the primitive.

Measured on localdev in Chrome: the audit log's Next button keeps the focus
on reaching the last page, reads aria-disabled, and the guarded click does
nothing.

frontend/src/components/ShareDialog.tsx

@@ -13,6 +13,7 @@ import { formatDate } from "../lib/format";
 import { api, type DownloadJob, type ShareLink, type ShareRecipient } from "../lib/api";
 import { inputCls } from "./ui/form";
 import { Button } from "./ui/Button";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 const EXPIRY_DAYS = [null, 1, 7, 30] as const;
 
 // --- share with a registered user (ACL) -----------------------------------------------------
@@ -70,7 +71,7 @@ function UserShare({ job }: { job: DownloadJob }) {
         {open && filtered.length > 0 && (
           <div className="absolute z-base mt-1 w-full rounded-lg glass-menu overflow-hidden">
             {filtered.map((u) => (
-              <button
+              <FocusSafeButton
                 key={u.id}
                 onClick={() => share.mutate(u.email)}
                 disabled={share.isPending}
@@ -78,7 +79,7 @@ function UserShare({ job }: { job: DownloadJob }) {
               >
                 <span className="truncate">{u.display_name || u.email}</span>
                 {u.display_name && <span className="text-xs text-muted truncate">{u.email}</span>}
-              </button>
+              </FocusSafeButton>
             ))}
           </div>
         )}
@@ -229,15 +230,15 @@ function LinkRow({ link, onChanged }: { link: ShareLink; onChanged: () => void }
             )}
             className="flex-1 min-w-0 bg-surface border border-border rounded-md px-2 py-1 text-xs focus:outline-hidden focus:ring-2 focus:ring-accent/40"
           />
-          <button
+          <FocusSafeButton
             onClick={() => savePass.mutate(newPass.trim())}
             disabled={savePass.isPending || !newPass.trim()}
             className="px-2 py-1 rounded-md text-xs font-semibold btn-primary disabled:opacity-40 transition"
           >
             {t("downloads.share.savePassword")}
-          </button>
+          </FocusSafeButton>
           {link.has_password && (
-            <button
+            <FocusSafeButton
               onClick={async () => {
                 if (
                   await confirm({
@@ -253,7 +254,7 @@ function LinkRow({ link, onChanged }: { link: ShareLink; onChanged: () => void }
               className="px-2 py-1 rounded-md text-xs text-muted hover:text-danger transition"
             >
               {t("downloads.share.removePassword")}
-            </button>
+            </FocusSafeButton>
           )}
         </div>
       )}