refactor(notifications): complete UX redesign

- Self-contained rules replace abstract channels
- Inline delivery config (broadcast/DM/email/webhook or none)
- quiet_hours_enabled master toggle separate from start/end times
- delivery_type="" valid: rule matches but does not deliver
- Severity dropdown with plain-English descriptions
- Example messages per alert category
- Default baseline rules: Emergency Broadcast, Infrastructure Down, Fire Alert, Severe Weather
- Condition vs Schedule trigger types
- Test and preview buttons per rule
- stream_flood_warning renamed from flood_warning (distinct from packet_flood)
- Categories display with descriptions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

meshai/config.py

@@ -448,7 +448,7 @@ class NotificationRuleConfig:
     custom_message: str = ""
 
     # Delivery type
-    delivery_type: str = "mesh_broadcast"  # mesh_broadcast, mesh_dm, email, webhook
+    delivery_type: str = ""  # mesh_broadcast, mesh_dm, email, webhook
 
     # Mesh broadcast fields
     broadcast_channel: int = 0
@@ -482,6 +482,7 @@ class NotificationsConfig:
     """Notification system settings."""
 
     enabled: bool = False
+    quiet_hours_enabled: bool = True  # Master toggle for quiet hours
     quiet_hours_start: str = "22:00"
     quiet_hours_end: str = "06:00"
     rules: list = field(default_factory=list)  # List of NotificationRuleConfig