feat: Make one-to-one chats read-only the first seconds of a SecureJoin (#5512)

This protects Bob (the joiner) of sending unexpected-unencrypted messages during an otherwise nicely
running SecureJoin.

If things get stuck, however, we do not want to block communication -- the chat is just
opportunistic as usual, but that needs to be communicated:
1. If Bob's chat with Alice is `Unprotected` and a SecureJoin is started, then add info-message
   "Establishing guaranteed end-to-end encryption, please wait..." and let `Chat::can_send()` return
   `false`.
2. Once the info-message "Messages are guaranteed to be e2ee from now on" is added, let
   `Chat::can_send()` return `true`.
3. If after SECUREJOIN_WAIT_TIMEOUT seconds `2.` did not happen, add another info-message "Could not
   yet establish guaranteed end-to-end encryption but you may already send a message" and also let
   `Chat::can_send()` return `true`.

Both `2.` and `3.` require the event `ChatModified` being sent out so that UI pick up the change wrt
`Chat::can_send()` (this is the same way how groups become updated wrt `can_send()` changes).

SECUREJOIN_WAIT_TIMEOUT should be 10-20 seconds so that we are reasonably sure that the app remains
active and receiving also on mobile devices. If the app is killed during this time then we may need
to do step 3 for any pending Bob-join chats (right now, Bob can only join one chat at a time).

src/constants.rs

@@ -223,6 +223,11 @@ pub(crate) const DC_BACKGROUND_FETCH_QUOTA_CHECK_RATELIMIT: u64 = 12 * 60 * 60;
 /// in the group membership consistency algo to reject outdated membership changes.
 pub(crate) const TIMESTAMP_SENT_TOLERANCE: i64 = 60;
 
+/// How long a 1:1 chat can't be used for sending while the SecureJoin is in progress. This should
+/// be 10-20 seconds so that we are reasonably sure that the app remains active and receiving also
+/// on mobile devices. See also [`crate::chat::CantSendReason::SecurejoinWait`].
+pub(crate) const SECUREJOIN_WAIT_TIMEOUT: u64 = 15;
+
 #[cfg(test)]
 mod tests {
     use num_traits::FromPrimitive;