fix(ui): dialogs, menus and busy buttons keep the keyboard focus where it belongs

Measured in Chrome before each change:

- A dialog opened from a menu lost its focus: the menu row ran its action
  and closed in one batch, and floating-ui's focus return (a microtask)
  moved the focus to the row BEHIND the confirm dialog's scrim 7 ms after
  it opened, where Enter activated the row. MenuItemList now closes first
  and runs the action a task later; the dialog also captures the row, not
  a dying menu item, as its restore target.
- A side panel opened as a flyout collapsed the moment a dialog opened
  from inside it, unmounting the row the dialog hands the focus back to.
  useHoverFocusWithin holds while the focus is inside an aria-modal dialog.
- ui/Button's loading state set native disabled; Chrome moved the focus of
  the pressed button to <body> and never gave it back. Loading is now
  aria-disabled with a click guard that also cancels implicit submission.
- Escape/Enter in the playlist rail's rename field dropped the focus on
  <body>. The row refocuses from its ref callback when it re-attaches (a
  post-commit focus sticks; the earlier attempt ran before the button
  existed).
- PlayerModal was the one aria-modal shell without the dialog contract:
  it is now named by its title, traps Tab, restores the focus to its
  opener and shares one counted body scroll lock with every dialog.

frontend/src/components/ui/ContextMenu.dom.test.tsx

@@ -108,13 +108,13 @@ describe("useContextMenu / ContextMenu", () => {
     expect(menuitems()).toEqual([]);
   });
 
-  it("selecting an item fires it and closes the menu", () => {
+  it("selecting an item closes the menu, then fires it (see ActionMenu's order test)", async () => {
     const onSelect = vi.fn();
     renderList(() => [{ id: "a", label: "Remove", onSelect }]);
     fireEvent.contextMenu(row("alpha"));
     fireEvent.click(item("Remove"));
-    expect(onSelect).toHaveBeenCalledTimes(1);
     expect(menuitems()).toEqual([]);
+    await vi.waitFor(() => expect(onSelect).toHaveBeenCalledTimes(1));
   });
 
   it("Shift+F10 opens it from the keyboard", () => {
@@ -217,7 +217,7 @@ describe("useContextMenu / ContextMenu", () => {
       expect(menuitems()).toEqual(["Play", "Move to", "Watch later", "Homelab"]);
     });
 
-    it("choosing inside a submenu closes the WHOLE stack, not just the submenu", () => {
+    it("choosing inside a submenu closes the WHOLE stack, not just the submenu", async () => {
       const chosen = vi.fn();
       renderList(() => [
         { id: "play", label: "Play", onSelect: vi.fn() },
@@ -226,8 +226,8 @@ describe("useContextMenu / ContextMenu", () => {
       fireEvent.contextMenu(row("alpha"));
       fireEvent.click(item("Move to"));
       fireEvent.click(item("Homelab"));
-      expect(chosen).toHaveBeenCalledTimes(1);
       expect(menuitems()).toEqual([]);
+      await vi.waitFor(() => expect(chosen).toHaveBeenCalledTimes(1));
     });
   });