feat: option to mute messaging-gateway chat notifications

feat: option to mute messaging-gateway chat notifications

Feature description

Conversations that arrive via messaging gateways (Discord, WhatsApp, Telegram)
generate in-app toasts, browser notifications, sidebar unread badges, and
tab-bar unread dots. When the user is already reading (and replying to) those
conversations in the platform UI itself, the mirrored gateway notifications
are pure noise.

Proposed solution

A user preference (toggle under Settings → Notifications) that silences
notification surfaces for gateway chats when the user reads them in the UI:

• no in-app toast, no browser notification
• no sidebar unread badge
• no tab-bar unread dot

The chats themselves remain fully visible and functional — only the
notification mirroring is suppressed.

Implementation notes

• Unread counting accepts an exclude_bridge flag, driven by the user's persisted preference.
• The chat's "done" socket event can carry a flag for gateway chats so the client can skip toasts for chats it hasn't loaded.

Additional context

Implemented on a fork; can share implementation notes.

Start by tracing the Settings → Notifications preference, unread counting with the exclude_bridge flag, and the chat "done" socket event described in the issue. Done means gateway chats can be read normally while their toasts, browser notifications, sidebar badges, and tab-bar dots are suppressed according to the persisted preference.