feat: MSC4306 thread subscription button and auto-subscribe behaviours

[nathanael-h]

Summary

• ThreadSubscriptionButton: Follow/Following button with bell icon in the thread view header, using MSC4306 PUT/DELETE/GET …/subscription endpoints via new matrix-js-sdk methods.
• Subscribe-on-send (Behaviour A): when the user sends a message in a thread, a fire-and-forget PUT /subscription (manual, body {}) is issued.
• Subscribe-on-mention (Behaviour B): when an incoming thread event triggers a notify push action, PUT /subscription with {"automatic": "$causeEventId"} is sent — unless the sender is ignored/banned or we've already subscribed this session. 409 M_CONFLICTING_UNSUBSCRIPTION is gracefully swallowed.
• In-session cache (autoSubscribedThreads Set + SDK-level cache) to avoid duplicate PUTs.

Depends on: matrix-org/matrix-js-sdk#5311
Ref: matrix-org/matrix-spec-proposals#4306

Test plan

• Open a thread → button shows "Follow" or "Following" based on subscription state
• Click "Follow" → PUT /subscription with {}, button toggles to "Following"
• Click "Following" → DELETE /subscription, button toggles to "Follow"
• Send a reply in a thread → network tab shows PUT /subscription with body {}
• Receive a mention in a thread from another user → network tab shows PUT /subscription with {"automatic":"$eventId"}
• Ignored/banned sender mention → no PUT fires
• Unfollow a thread, receive a mention → 409 swallowed, button stays "Follow"