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/ui/Button.tsx

@@ -2,6 +2,7 @@ import { forwardRef } from "react";
 import type { ButtonHTMLAttributes, ReactNode, RefAttributes } from "react";
 import clsx from "clsx";
 import { Loader2 } from "lucide-react";
+import { FocusSafeButton } from "./FocusSafeButton";
 
 // The ONE action-button primitive. Before this there were 385 raw <button>s styling themselves with
 // ad-hoc utility strings over the CSS paint-recipes (.btn-primary/.btn-danger, index.css) + the btnCls
@@ -142,10 +143,14 @@ const ButtonImpl = forwardRef<HTMLButtonElement, ButtonOwnProps>(function Button
     disabled,
     type,
     onClick,
+    onFocus,
+    onBlur,
     ...rest
   },
   ref,
 ) {
+  // Unavailable while busy (`loading`) or disabled — without dropping a focused button's focus:
+  // see FocusSafeButton, which owns that rule for every button in the app.
   // A link must not inherit the action-button padding/radius, so its size default differs. Resolved
   // here rather than by asking every call site to pass `size="none"` alongside `variant="link"` —
   // a pairing everyone would eventually forget, which is how a "link" grows a button's box.
@@ -157,26 +162,14 @@ const ButtonImpl = forwardRef<HTMLButtonElement, ButtonOwnProps>(function Button
   const touchTarget = variant !== "link";
   const leading = loading ? <Loader2 className="w-4 h-4 animate-spin" /> : icon;
   return (
-    <button
+    <FocusSafeButton
       ref={ref}
       type={type ?? "button"}
-      // `loading` blocks activation WITHOUT native `disabled`. A focused button that turns disabled
-      // loses the focus: Chrome moved it to <body> ~60 ms after the reroll click on Home and nothing
-      // handed it back when the button re-enabled (measured, E11 S2) — so a keyboard user was dropped
-      // out of the page on every press. `aria-disabled` keeps the button focusable and announced as
-      // unavailable; the click guard is what actually stops the double-submit, whatever `disabled`
-      // says, and its preventDefault also cancels a form's implicit submission (which is a click on
-      // the default button).
       disabled={disabled}
-      aria-disabled={loading || undefined}
-      aria-busy={loading || undefined}
-      onClick={(e) => {
-        if (loading) {
-          e.preventDefault();
-          return;
-        }
-        onClick?.(e);
-      }}
+      busy={loading}
+      onClick={onClick}
+      onFocus={onFocus}
+      onBlur={onBlur}
       aria-label={label}
       title={label}
       className={clsx(
@@ -191,7 +184,7 @@ const ButtonImpl = forwardRef<HTMLButtonElement, ButtonOwnProps>(function Button
     >
       {leading}
       {!iconOnly && children}
-    </button>
+    </FocusSafeButton>
   );
 });