Remember last used topic when switching away and back to a chat #1449

Feature Description

When leaving a topic-enabled chat and later returning to that chat, reopen the last topic that was active — not the topic list / “All messages” overview / General.

Motivation

Topic-enabled chats (forum groups and topic-enabled bot DMs) currently drop you on the no-topic / overview surface after you switch to another chat and come back. You have to find and re-open the topic you were just using. With many topics this is slow, and it is easy to send the next message into the wrong place (or not be able to send at all from the overview).

Drafts inside topics are already a known gap (#1356). This request is about which topic is selected on re-entry, not about draft text.

Proposed Solution

Per chat, persist the last viewed message_thread_id.

• Switching to another chat and back restores that topic
• App relaunch should restore it as well, if the topic still exists
• If the last topic was deleted, fall back to the topic list
• Optional: a setting “Always open last topic” vs “Always open topic list”

Alternatives Considered

• Pinning the current topic — still a click, and pins are for importance, not recency
• ⌘K — does not restore last topic, and in topic chats often opens the mixed feed (Keyboard navigation (⌘K) into groups with topics enabled #1262)

Additional Context

Telegram for macOS. Same pain exists when using a bot DM with Topics enabled and many parallel threads.