refine call states (#7179)

- sync declined calls from callee to caller, as usual in all larger
messengers
- introduce the call states "Missed call", "Declined call" and
"Cancelled all" ("Ended call" is gone)
- allow calling end_call()/accept_call() for already ended/accepted
calls, in practise, handling all cornercases is tricky in UI - and the
state needs anyways to be tracked.
- track and show the call duration

the duration calculation depends on local time, but it is displayed only
coarse and is not needed for any state. this can be improved as needed,
timestamps of the corresponding messages are probably better at some
point. or ending device sends its view of the time around. but for the
first throw, it seems good enough

if we finally want that set of states, it can be exposed to a json-info
in a subsequent call, so that the UI can render it more nicely. fallback
strings as follows will stay for now to make adaption in other UI easy,
and for debugging:

<img width="320" alt="IMG_0154"
src="https://github.com/user-attachments/assets/09a89bfb-66f4-4184-b05c-e8040b96cf44"
/>

successor of https://github.com/chatmail/core/pull/6650

src/sync.rs

@@ -71,9 +71,6 @@ pub(crate) enum SyncData {
     DeleteMessages {
         msgs: Vec<String>, // RFC724 id (i.e. "Message-Id" header)
     },
-    RejectIncomingCall {
-        msg: String, // RFC724 id (i.e. "Message-Id" header)
-    },
 }
 
 #[derive(Debug, Serialize, Deserialize)]
@@ -267,7 +264,6 @@ impl Context {
                     SyncData::Config { key, val } => self.sync_config(key, val).await,
                     SyncData::SaveMessage { src, dest } => self.save_message(src, dest).await,
                     SyncData::DeleteMessages { msgs } => self.sync_message_deletion(msgs).await,
-                    SyncData::RejectIncomingCall { msg } => self.sync_call_rejection(msg).await,
                 },
                 SyncDataOrUnknown::Unknown(data) => {
                     warn!(self, "Ignored unknown sync item: {data}.");