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/Welcome.tsx

@@ -35,6 +35,7 @@ import { inputCls } from "./ui/form";
 import { entryHash, entrySearch } from "../lib/entryUrl";
 import type { VerifyEntry } from "../lib/useVerifyEntry";
 import { MODE_URL, type Mode } from "./welcomeMode";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 const EMAIL_RE = /^[^@\s]+@[^@\s]+\.[^@\s]+$/;
 const PASSWORD_MIN = 10;
@@ -682,13 +683,13 @@ function AuthCard({ verify }: { verify: VerifyEntry }) {
                 {error}
               </p>
             )}
-            <button
+            <FocusSafeButton
               type="submit"
               disabled={busy}
               className="px-4 py-2.5 rounded-xl font-semibold btn-primary disabled:opacity-50 transition"
             >
               {busy ? t("welcome.auth.working") : t(SUBMIT_KEY[mode])}
-            </button>
+            </FocusSafeButton>
           </form>
 
           {/* A confirmation mail can be slow, filtered, or — before E9.5 S6 — never sent at all for a
@@ -772,13 +773,13 @@ function AuthCard({ verify }: { verify: VerifyEntry }) {
                       placeholder={t("welcome.auth.demoEmail")}
                       className={inputCls}
                     />
-                    <button
+                    <FocusSafeButton
                       type="submit"
                       disabled={demoBusy}
                       className="shrink-0 px-3 py-2.5 rounded-xl text-sm font-semibold bg-card border border-border hover:border-accent disabled:opacity-50 transition"
                     >
                       {demoBusy ? t("welcome.auth.working") : t("welcome.auth.demoEnter")}
-                    </button>
+                    </FocusSafeButton>
                   </div>
                   {demoError && <p className="text-xs text-danger">{demoError}</p>}
                 </form>