feat: Show broadcast channels in their own, proper "Channel" chat (#6901)

Part of #6884 

----

- [x] Add new chat type `InBroadcastChannel` and `OutBroadcastChannel`
for incoming / outgoing channels, where the former is similar to a
`Mailinglist` and the latter is similar to a `Broadcast` (which is
removed)
- Consideration for naming: `InChannel`/`OutChannel` (without
"broadcast") would be shorter, but less greppable because we already
have a lot of occurences of `channel` in the code. Consistently calling
them `BcChannel`/`bc_channel` in the code would be both short and
greppable, but a bit arcane when reading it at first. Opinions are
welcome; if I hear none, I'll keep with `BroadcastChannel`.
- [x] api: Add create_broadcast_channel(), deprecate
create_broadcast_list() (or `create_channel()` / `create_bc_channel()`
if we decide to switch)
  - Adjust code comments to match the new behavior.
- [x] Ask Desktop developers what they use `is_broadcast` field for, and
whether it should be true for both outgoing & incoming channels (or look
it up myself)
- I added `is_out_broadcast_channel`, and deprecated `is_broadcast`, for
now
- [x] When the user changes the broadcast channel name, immediately show
this change on receiving devices
- [x] Allow to change brodacast channel avatar, and immediately apply it
on the receiving device
- [x] Make it possible to block InBroadcastChannel
- [x] Make it possible to set the avatar of an OutgoingChannel, and
apply it on the receiving side
- [x] DECIDE whether we still want to use the broadcast icon as the
default icon or whether we want to use the letter-in-a-circle
- We decided to use the letter-in-a-circle for now, because it's easier
to implement, and I need to stay in the time plan
- [x] chat.rs: Return an error if the user tries to modify a
`InBroadcastChannel`
- [x] Add automated regression tests
- [x] Grep for `broadcast` and see whether there is any other work I
need to do
- [x] Bug: Don't show `~` in front of the sender's same in broadcast
lists

----

Note that I removed the following guard:

```rust
        if !new_chat_contacts.contains(&ContactId::SELF) {
            warn!(
                context,
                "Received group avatar update for group chat {} we are not a member of.", chat.id
            );
        } else if !new_chat_contacts.contains(&from_id) {
            warn!(
                context,
                "Contact {from_id} attempts to modify group chat {} avatar without being a member.",
                chat.id,
            );
        } else [...]
```

i.e. with this change, non-members will be able to modify the avatar.
Things were slightly easier this way, and I think that this is in line
with non-members being able to modify the group name and memberlist
(they need to know the Group-Chat-Id, anyway), but I can also change it
back.

deltachat-jsonrpc/src/api.rs

@@ -926,7 +926,7 @@ impl CommandApi {
     ///   explicitly as it may happen that oneself gets removed from a still existing
     ///   group
     ///
-    /// - for broadcasts, all recipients are returned, DC_CONTACT_ID_SELF is not included
+    /// - for broadcast channels, all recipients are returned, DC_CONTACT_ID_SELF is not included
     ///
     /// - for mailing lists, the behavior is not documented currently, we will decide on that later.
     ///   for now, the UI should not show the list for mailing lists.
@@ -975,18 +975,30 @@ impl CommandApi {
             .map(|id| id.to_u32())
     }
 
-    /// Create a new broadcast list.
-    ///
-    /// Broadcast lists are similar to groups on the sending device,
-    /// however, recipients get the messages in a read-only chat
-    /// and will see who the other members are.
-    ///
-    /// For historical reasons, this function does not take a name directly,
-    /// instead you have to set the name using dc_set_chat_name()
-    /// after creating the broadcast list.
+    /// Deprecated 2025-07 in favor of create_broadcast().
     async fn create_broadcast_list(&self, account_id: u32) -> Result<u32> {
+        self.create_broadcast(account_id, "Channel".to_string())
+            .await
+    }
+
+    /// Create a new **broadcast channel**
+    /// (called "Channel" in the UI).
+    ///
+    /// Broadcast channels are similar to groups on the sending device,
+    /// however, recipients get the messages in a read-only chat
+    /// and will not see who the other members are.
+    ///
+    /// Called `broadcast` here rather than `channel`,
+    /// because the word "channel" already appears a lot in the code,
+    /// which would make it hard to grep for it.
+    ///
+    /// After creation, the chat contains no recipients and is in _unpromoted_ state;
+    /// see [`CommandApi::create_group_chat`] for more information on the unpromoted state.
+    ///
+    /// Returns the created chat's id.
+    async fn create_broadcast(&self, account_id: u32, chat_name: String) -> Result<u32> {
         let ctx = self.get_context(account_id).await?;
-        chat::create_broadcast_list(&ctx)
+        chat::create_broadcast(&ctx, chat_name)
             .await
             .map(|id| id.to_u32())
     }