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/plexViews.tsx

@@ -738,7 +738,7 @@ export function PlexSeasonView({
               />
             }
           >
-            <Button variant="link-muted" onClick={onBack} className="text-sm">
+            <Button variant="link-muted" selectable onClick={onBack} className="text-sm">
               {d.show.title}
             </Button>
             <PageHeading className="text-2xl font-semibold leading-tight">{se.title}</PageHeading>