Add the multi-account announcement screen (#6263)

* Add the multi-account announcement screen

A self-sizing bottom sheet telling users they can now add multiple accounts, matching the designs. Close and Add account both report back to the coordinator; nothing presents it yet.

* Use the Localazy strings for the multi-account announcement

The copy now lives in Localazy as screen.multi_account_announcement.*, named after the screen rather than the room list.

* Apply review suggestions to the multi-account announcement

- Use an ElementNavigationStack with a toolbar close button, like RedactConfirmationView, instead of an overlaid glass button.

* Remove the large text preview from the multi-account announcement

Other sheets built the same way don't snapshot Dynamic Type sizes, and the iOS 26 bar material it would show isn't rendered in snapshots.

* Use TitleAndIcon for the multi-account announcement

TitleAndIcon now takes an attributed subtitle (#6274), so the sheet no longer needs to rebuild its layout by hand. The snapshots are unchanged.

AccessibilityTests/Sources/GeneratedAccessibilityTests.swift

@@ -391,6 +391,10 @@ extension AccessibilityTests {
         try await performAccessibilityAudit(named: "MessageText_Previews")
     }
 
+    func testMultiAccountAnnouncementScreen() async throws {
+        try await performAccessibilityAudit(named: "MultiAccountAnnouncementScreen_Previews")
+    }
+
     func testNoticeRoomTimelineView() async throws {
         try await performAccessibilityAudit(named: "NoticeRoomTimelineView_Previews")
     }