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/UploadDialog.tsx

@@ -12,6 +12,7 @@ import { notify } from "../lib/notifications";
 import { navigateTo } from "../lib/nav";
 import { formatBytes } from "../lib/format";
 import { inputCls } from "./ui/form";
+import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // Local-file ingest (E7 S5c · D5-A). A general upload — ANY file type, no format list: media
 // (video/audio) joins the library with the full function set, a generic file is download-only. The
@@ -75,7 +76,7 @@ export default function UploadDialog({ onClose }: { onClose: () => void }) {
       <p className="text-sm text-muted mb-4">{t("downloads.upload.hint")}</p>
 
       {/* Drop zone doubles as the picker trigger — a click anywhere opens the native file dialog. */}
-      <button
+      <FocusSafeButton
         type="button"
         onClick={pick}
         onDragEnter={(e) => {
@@ -117,7 +118,7 @@ export default function UploadDialog({ onClose }: { onClose: () => void }) {
             </span>
           </>
         )}
-      </button>
+      </FocusSafeButton>
       <input
         ref={inputRef}
         type="file"