feat(design): titles in click surfaces stay selectable (E28 S3e4b)

A button whose text is a title or a name is now `selectable` (FocusSafeButton,
under Pressable and Button): Chrome selects inside a button once it is
user-select: text (measured), and the click that ends a selection, or a press
dragged past the slop, opens nothing. 23 surfaces carry it (cards, rows, people,
channels, playlists, users, views, messages, log lines, description links); the
Plex caption div gets the same guard. A contract test pins every plain Pressable
with its reason, so a new one has to choose.

frontend/src/components/ui/Button.tsx

@@ -36,6 +36,8 @@ interface ButtonOwnProps extends Omit<ButtonHTMLAttributes<HTMLButtonElement>, "
    *  `media` variant. */
   round?: boolean;
   fullWidth?: boolean;
+  /** Its text is a title or a name (a link back to a show): it stays selectable (FocusSafeButton). */
+  selectable?: boolean;
 }
 
 // A button of this primitive always renders text, or it is an `IconButton`. `<Button icon={x} />`