feat: add "Include Specials" toggle to the show download dialog

The aired-order rework can sweep correctly-placed Specials into "download
next N"; this toggle lets users opt out. Shown only for whole shows
(reusing FocusableSwitchListTile via an optional toggle on the shared
option-picker dialog), remembered across opens via a BoolPref
(default on = unchanged behavior). Filters Specials at the single
collect choke point (_collectPlayable), with a guard so explicitly
downloading the Specials season still queues its episodes.

lib/i18n/bg.i18n.json

@@ -1031,6 +1031,7 @@
     "unwatchedOnly": "Само негледани",
     "nextNUnwatched": "Следващите ${count} негледани",
     "customAmount": "Персонален брой...",
+    "includeSpecials": "Включи специалните",
     "howManyEpisodes": "Колко епизода?",
     "itemsQueued": "${count} елемента са добавени в опашката за изтегляне",
     "keepSynced": "Поддържай синхронизирано",