feat(richtext): persist rich-text composer drafts across attachment-menu sessions

Store the rich-text editor's document/media/emoji as a Codable draft in the
item cache keyed by peerId+threadId: load on attachment-menu open, save on
syncContent, clear on send. Media/TelegramMediaFile (not Codable) round-trip as
self-describing root objects. Adds Namespaces.CachedItemCollection.richTextComposerDrafts
and reserves ApplicationSpecificItemCacheCollectionId.richTextArticleDrafts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>