📣 Android: Announcement and Channel presets

[nickvergessen]

• Overview 📣 Overview: Announcement and Channel presets spreed#18683

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Overview

Channel (channel preset, attribute 8) — broadcasting to a potentially huge audience:

• Only moderators can post, everybody else can only react.
• Permissions are a default, so individuals can still be granted the chat permission.
• Calls are impossible for everyone, including moderators.
• The participants list is only available to moderators.
• System messages are not shown (client-side only).
• Listable for users by default.

Announcement (announcement preset, attributes 8|16 = 24) — a channel with extra rules:

• Everything a channel does, unless contradicted below.
• Not listable by default.
• Non-moderators can not leave; moderators can, and can still remove others.
• Notifies about all messages by default (changeable afterwards).

Capability announcement-preset indicates the existence and support of both presets. A conversation is a channel when TalkRoom.attributes & 8 (the CHANNEL bit) is set, and an announcement when TalkRoom.attributes & 16 (the ANNOUNCEMENT bit) is set. An announcement is a channel with additional restrictions, so it always has both bits set — every channel behavior below therefore applies to announcements as well, and attributes & 8 is the only check needed for the shared parts.

Detection

• Detect channels via TalkRoom.attributes & 8 and announcements via TalkRoom.attributes & 16; only apply this behavior when the announcement-preset capability is present.
• Add channel and announcement to CONVERSATION.PRESET in src/constants.ts and give them an icon in presetIcons (NewConversationSetupPage.vue).

Channels (also applies to announcements)

• Do not render system messages in channel conversations. (frontend-only, no backend block)
• Hide the participants list / tab for non-moderators — the list can be enormous. (backend returns 403 on GET /room/{token}/participants, so do not call it)
• Hide the call buttons and the whole call UI. Calls are impossible for everyone, including moderators: the backend strips PERMISSIONS_CALL_START/PERMISSIONS_CALL_JOIN from permissions and reports canStartCall: false, and joining a call returns 403.
• Show that only moderators can post: non-moderators get permissions without PERMISSIONS_CHAT (128) but with PERMISSIONS_REACT (256), so the message composer should be replaced by a reaction-only affordance. Individual participants can still be granted the chat permission, so always follow the actual permissions value rather than assuming.

Announcements only

• Hide the "Leave conversation" action for non-moderators — canLeaveConversation is false for them and DELETE /room/{token}/participants/self returns 400 announcement. Moderators can still leave and can still remove other participants.
• The notification level defaults to "all messages" (1) for users added to an announcement. It is only a default, so the notification settings must stay editable.