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/en.i18n.json

@@ -1040,6 +1040,7 @@
     "unwatchedOnly": "Unwatched only",
     "nextNUnwatched": "Next ${count} unwatched",
     "customAmount": "Custom amount...",
+    "includeSpecials": "Include Specials",
     "howManyEpisodes": "How many episodes?",
     "itemsQueued": "${count} items queued for download",
     "keepSynced": "Keep synced",