Add Collections notification support

[joebarbere]

Phase 2 of #2478, following #2479 (read-only viewing).

Mastodon 4.6 added two notification types for Collections: added_to_collection (someone added you to their collection) and collection_update (a collection you're featured in changed). Both carry the Collection entity on the notification.

Until now Ice Cubes dropped these silently — an unrecognized type maps to a nil supportedType, and both consolidation paths skip those notifications entirely.

What's here

• Two new NotificationType cases, and collection decoded on both Notification and NotificationGroup.
• The collection is carried through v1 consolidation, the v2 grouped mapping, the streaming path, and the v2 group merge (newGroup.collection ?? existingGroup.collection, so a merge can't drop it).
• NotificationRowCollectionView renders the same bordered pill used on profiles, tapping through to the existing CollectionDetailView. Icons are rectangle.stack.badge.person.crop.fill and person.2.crop.square.stack.fill, tinted indigo.
• Push-notification taps route to the collection as well.
• Both types appear in the notifications filter menu, gated on a new CurrentInstance.isCollectionsSupported.

Notes

NotificationGroup carries collection too. The docs page for that entity doesn't list it, but REST::NotificationGroupSerializer emits it for these types alongside REST::NotificationSerializer — I went with the source, so the grouped (v2) path works and not just v1.

isCollectionsSupported checks api_versions.mastodon >= 10 rather than parsing the version string. Collections shipped in 4.6.0, which bumped that to 10 (4.5.0 was 7, 4.7.1 is 11). The existing version helper does Float(String.prefix(3)), which would read a future "4.10" as 4.1 — this follows isQuoteSupported instead.

There are no WebPush alerts keys for collections, so the server never pushes these types; the NotificationTab branch exists for exhaustiveness and future-proofing.

Testing

Builds clean for the iOS simulator. ModelsTests passes 16/16, including four new decode tests: both notification types, a notification with no collection attached, and a NotificationGroup carrying one.