Notifications: opt-in per-post OS banner class and per-channel sounds

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

v1 per-channel notification levels deliberately add no per-post OS banners (VISION.md: zero-by-default) — level "All new posts" is exactly today's behavior. Slack parity eventually wants:

- An opt-in per-channel notification class that raises ordinary posts to OS banners (desktop + mobile), layered on the existing `notifyDecisionForEvent` ladder as a delivery-surface concern (SoC: the ladder decides *whether*, the class decides *how loud*).
- Per-channel notification sounds.

Both are additive fields on the `channel-notify-prefs` schema (`docs/nips/NIP-CN.md`), which round-trips unknown fields, so older clients are unaffected.

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