feat: Sync Config::Displayname across devices (#4893)

We already synchronise status/footer when we see a self-sent message with a Chat-Version
header. Would be nice to do the same for display name.

But let's do it the same way as for `Config::{MdnsEnabled,ShowEmails}`. Otherwise, if we sync the
display name using the "From" header, smth like `Param::StatusTimestamp` is needed then to reject
outdated display names. Also this timestamp needs to be updated when `Config::Displayname` is set
locally. Also this wouldn't work if system time isn't synchronised on devices. Also using multiple
approaches to sync different config values would lead to more code and bugs while having almost no
value -- using "From" only saves some bytes and allows to sync some things w/o the synchronisation
itself to be enabled. But the latter also can be a downside -- if it's usual synchonisation, you can
(potentially) disable it and share the same email account across people in some organisation
allowing them to have different display names. With using "From" for synchronisation such a
capability definitely requires a new config option.

src/config.rs

@@ -351,7 +351,10 @@ impl Config {
     /// This must be checked on both sides so that if there are different client versions, the
     /// synchronisation of a particular option is done or not done both ways.
     pub(crate) fn is_synced(&self) -> bool {
-        matches!(self, Self::MdnsEnabled | Self::ShowEmails)
+        matches!(
+            self,
+            Self::Displayname | Self::MdnsEnabled | Self::ShowEmails
+        )
     }
 }
 
@@ -482,8 +485,9 @@ impl Context {
         &self,
         sync: sync::Sync,
         key: Config,
-        value: Option<&str>,
+        mut value: Option<&str>,
     ) -> Result<()> {
+        let better_value;
         match key {
             Config::Selfavatar => {
                 self.sql
@@ -510,10 +514,11 @@ impl Context {
                 ret?
             }
             Config::Displayname => {
-                let value = value.map(improve_single_line_input);
-                self.sql
-                    .set_raw_config(key.as_ref(), value.as_deref())
-                    .await?;
+                if let Some(v) = value {
+                    better_value = improve_single_line_input(v);
+                    value = Some(&better_value);
+                }
+                self.sql.set_raw_config(key.as_ref(), value).await?;
             }
             Config::Socks5Enabled
             | Config::BccSelf
@@ -911,6 +916,15 @@ mod tests {
         sync(&alice0, &alice1).await?;
         assert!(alice1.get_config_bool(Config::MdnsEnabled).await?);
 
+        alice0
+            .set_config(Config::Displayname, Some("Alice Sync"))
+            .await?;
+        sync(&alice0, &alice1).await?;
+        assert_eq!(
+            alice1.get_config(Config::Displayname).await?,
+            Some("Alice Sync".to_string())
+        );
+
         Ok(())
     }
 }

src/receive_imf.rs

@@ -388,7 +388,10 @@ pub(crate) async fn receive_imf_inner(
 ///
 /// Also returns whether it is blocked or not and its origin.
 ///
-/// * `prevent_rename`: passed through to `add_or_lookup_contacts_by_address_list()`
+/// * `prevent_rename`: if true, the display_name of this contact will not be changed. Useful for
+/// mailing lists: In some mailing lists, many users write from the same address but with different
+/// display names. We don't want the display name to change every time the user gets a new email from
+/// a mailing list.
 ///
 /// Returns `None` if From field does not contain a valid contact address.
 pub async fn from_field_to_contact_id(
@@ -2582,11 +2585,6 @@ pub(crate) async fn get_prefetch_parent_message(
 /// Looks up contact IDs from the database given the list of recipients.
 ///
 /// Returns vector of IDs guaranteed to be unique.
-///
-/// * param `prevent_rename`: if true, the display_name of this contact will not be changed. Useful for
-/// mailing lists: In some mailing lists, many users write from the same address but with different
-/// display names. We don't want the display name to change every time the user gets a new email from
-/// a mailing list.
 async fn add_or_lookup_contacts_by_address_list(
     context: &Context,
     address_list: &[SingleInfo],