Mobile: per-channel notification settings parity + push suppression

[LordMelkor]

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

Bring per-channel notification settings to the Flutter app:

• Read/write the channel-notify-prefs kind-30078 blob (spec: docs/nips/NIP-CN.md on the feat(desktop): Slack-parity per-channel notification settings #3233 branch). The schema already reserves a mobile per-channel field; the desktop client preserves unknown fields round-trip, so mobile can adopt incrementally.
• Mute/level UI in the channel long-press sheet mirroring desktop (All new posts / Just mentions / Mute and hide, timed mute presets).
• Push suppression: a muted or mentions-level channel must also stop mobile pushes. Pushes are relay-driven (NIP-PL leases), so the client should project its resolved per-channel decisions into its lease/subscription state (client-authored lease projection) rather than the relay learning to decrypt prefs.
• Keep the legacy channel-mutes dual-write interop working (newer-updatedAt wins per dimension is resolved client-side; see NIP-CN §Legacy interop).

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