fix(ui): show favorites section based on the current folder

The visibility of the favorites section was driven by the unified
inbox's starred envelopes for regular folders too, while the section
itself renders the current folder's starred messages. Folders with
starred mail hid the section until something populated the unified
inbox list, and folders without starred mail showed an empty section
once it was populated.

Assisted-by: Claude:claude-fable-5
Signed-off-by: Christoph Wurst <1374172+ChristophWurst@users.noreply.github.com>

Author: ChristophWurst

src/components/MailboxThread.vue

@@ -355,8 +355,9 @@ export default {
 			if (!this.sortFavorites) {
 				return false
 			}
+			const mailbox = this.mailbox.isPriorityInbox ? this.unifiedInbox : this.mailbox
 			const envelopes = this.mainStore.getEnvelopes(
-				this.unifiedInbox.databaseId,
+				mailbox.databaseId,
 				this.appendToSearch(this.favoriteQuery),
 			)
 			return envelopes.length > 0