Add native push notifications with reply actions, content preview, and configurable gateway

[eleboucher]

Adds native push notification support addressing #1176, #1183, #1209, and #1182.

Reply actions (#1209)

• Inline reply from OS notifications (Android, iOS, desktop)
• Action types registered in tauri.conf.json (sable-message / sable-reply)
• Reply queue with dedup and expiry (nativeNotificationReplies.ts)
• NativeNotificationActionRouting component wired into ClientNonUIFeatures

Content preview (#1182)

• Rich push payloads enabled by default (useRichPushPayloads)
• Fetch + decrypt event when not in local timeline (encrypted rooms)
• largeBody for single-message BigTextStyle (expanded view shows body)
• Sender name fallback for DMs when room name is missing

Configurable gateway URL (#1183)

• pushNotifyUrlOverride setting
• PushPusherConfig.ts helper for URL resolution + payload format

Distributor timeout (#1176)

• Bumps tauri-plugin-notifications to e7a8796 (fixes 2+ distributor selection + native warm-path notifications)

[eleboucher]

Consolidating into #1238