feat: Resend the last 10 messages to new broadcast member (#8151)

Last 10 messages in a broadcast channel are resent. They are sent and encrypted only to the new member, not to other subscribers.

Close #7678

Based on https://github.com/chatmail/core/pull/7854, with the following
changes:

- Refactor and simplify code, don't reuse the existing `get_chat_msgs()`
function
- Document that Param::Arg4 is also used for resent messages
cc818d9099
- It's unclear how exactly to resend webxdc status updates. After
discussing with @r10s, don't resend webxdc's at all for now.
38d57ebb30
- Don't set fake `msg_id` in resent messages
e7d0687d90
Setting the msg_id to `u32::MAX` is hacky, and may just as well break
    things as it may fix things, because some code may use the msg.id to
    load information from the database, like `get_iroh_topic_for_msg()`.
    From reading the code, I couldn't find any problem with leaving the
    correct `msg_id`, and if there is one, then we should add a function
parameter `is_resending` that is checked in the corresponding places.

Easiest to review file-by-file rather than individual commits, probably.
I'll squash-merge this.

---------

Co-authored-by: iequidoo <dgreshilov@gmail.com>

src/mimefactory.rs

@@ -194,6 +194,7 @@ fn new_address_with_name(name: &str, address: String) -> Address<'static> {
 }
 
 impl MimeFactory {
+    /// Returns `MimeFactory` for rendering `msg`.
     #[expect(clippy::arithmetic_side_effects)]
     pub async fn from_msg(context: &Context, msg: Message) -> Result<MimeFactory> {
         let now = time();
@@ -2226,18 +2227,18 @@ fn should_encrypt_symmetrically(msg: &Message, chat: &Chat) -> bool {
 /// rather than all recipients.
 /// This function returns the fingerprint of the recipient the message should be sent to.
 fn must_have_only_one_recipient<'a>(msg: &'a Message, chat: &Chat) -> Option<Result<&'a str>> {
-    if chat.typ == Chattype::OutBroadcast
-        && matches!(
-            msg.param.get_cmd(),
-            SystemMessage::MemberRemovedFromGroup | SystemMessage::MemberAddedToGroup
-        )
-    {
-        let Some(fp) = msg.param.get(Param::Arg4) else {
-            return Some(Err(format_err!("Missing removed/added member")));
-        };
-        return Some(Ok(fp));
+    if chat.typ != Chattype::OutBroadcast {
+        None
+    } else if let Some(fp) = msg.param.get(Param::Arg4) {
+        Some(Ok(fp))
+    } else if matches!(
+        msg.param.get_cmd(),
+        SystemMessage::MemberRemovedFromGroup | SystemMessage::MemberAddedToGroup
+    ) {
+        Some(Err(format_err!("Missing removed/added member")))
+    } else {
+        None
     }
-    None
 }
 
 async fn build_body_file(context: &Context, msg: &Message) -> Result<MimePart<'static>> {