feat: Delete messages from a chatmail server immediately by default (#5805) (#5840)

I.e. treat `DeleteServerAfter == None` as "delete at once". But when a backup is exported, set
`DeleteServerAfter` to 0 so that the server decides when to delete messages, in order not to break
the multi-device case. Even if a backup is not aimed for deploying more devices, `DeleteServerAfter`
must be set to 0, otherwise the backup is half-useful because after a restoration the user wouldn't
see new messages deleted by the device after the backup was done. But if the user explicitly set
`DeleteServerAfter`, don't change it when exporting a backup. Anyway even for non-chatmail case the
app should warn the user before a backup export if they have `DeleteServerAfter` enabled.

Also do the same after a backup import. While this isn't reliable as we can crash in between, this
is a problem only for old backups, new backups already have `DeleteServerAfter` set if necessary.

---------

Co-authored-by: Hocuri <hocuri@gmx.de>

src/config.rs

@@ -174,12 +174,12 @@ pub enum Config {
     /// Timer in seconds after which the message is deleted from the
     /// server.
     ///
-    /// Equals to 0 by default, which means the message is never
-    /// deleted.
+    /// 0 means messages are never deleted by Delta Chat.
     ///
     /// Value 1 is treated as "delete at once": messages are deleted
     /// immediately, without moving to DeltaChat folder.
-    #[strum(props(default = "0"))]
+    ///
+    /// Default is 1 for chatmail accounts before a backup export, 0 otherwise.
     DeleteServerAfter,
 
     /// Timer in seconds after which the message is deleted from the
@@ -470,10 +470,15 @@ impl Context {
         }
 
         // Default values
-        match key {
-            Config::ConfiguredInboxFolder => Ok(Some("INBOX".to_owned())),
-            _ => Ok(key.get_str("default").map(|s| s.to_string())),
-        }
+        let val = match key {
+            Config::ConfiguredInboxFolder => Some("INBOX"),
+            Config::DeleteServerAfter => match Box::pin(self.is_chatmail()).await? {
+                false => Some("0"),
+                true => Some("1"),
+            },
+            _ => key.get_str("default"),
+        };
+        Ok(val.map(|s| s.to_string()))
     }
 
     /// Returns Some(T) if a value for the given key exists and was successfully parsed.
@@ -555,11 +560,16 @@ impl Context {
     /// `None` means never delete the message, `Some(0)` means delete
     /// at once, `Some(x)` means delete after `x` seconds.
     pub async fn get_config_delete_server_after(&self) -> Result<Option<i64>> {
-        match self.get_config_int(Config::DeleteServerAfter).await? {
-            0 => Ok(None),
-            1 => Ok(Some(0)),
-            x => Ok(Some(i64::from(x))),
-        }
+        let val = match self
+            .get_config_parsed::<i64>(Config::DeleteServerAfter)
+            .await?
+            .unwrap_or(0)
+        {
+            0 => None,
+            1 => Some(0),
+            x => Some(x),
+        };
+        Ok(val)
     }
 
     /// Gets the configured provider, as saved in the `configured_provider` value.