Notifications: synced global default preferences

Follow-up to #3160 (desktop implementation: #3233).

The channel sheet's "Edit default preferences" links to Settings → Notifications, which is device-local today. Slack parity wants workspace-wide *synced* defaults (default level, default broadcasts opt-in, follow-new-threads) that per-channel entries override.

Natural shape: a `defaults` object in the same `channel-notify-prefs` kind-30078 blob (own `updatedAt`, same LWW merge), with the resolver falling back explicit-entry → synced default → built-in "all". Needs a migration story for existing device-local settings.

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