feat(dashboard): alerts page + toast notifications + polish

- Full Alerts page with active alerts, history table, subscriptions
- Active alert cards with severity styling and acknowledge button
- Alert history table with type/severity filtering and pagination
- Subscription viewer showing mesh subscriptions
- ToastProvider for app-wide toast notifications
- Toast notifications triggered on WebSocket alert_fired messages
- Auto-dismiss toasts after 8 seconds, click to navigate
- Page titles on all pages (Dashboard/Mesh/Environment/Config/Alerts)
- Improved alert_routes.py with proper pending alert handling
- Added AlertHistoryItem, Subscription types to api.ts
- Added fetchAlertHistory, fetchSubscriptions functions

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

dashboard-frontend/src/lib/api.ts

@@ -93,6 +93,34 @@ export interface Alert {
   scope_value?: string
 }
 
+export interface AlertHistoryItem {
+  id?: number
+  type: string
+  severity: string
+  message: string
+  timestamp: string
+  duration?: number
+  scope_type?: string
+  scope_value?: string
+  resolved_at?: string
+}
+
+export interface AlertHistoryResponse {
+  items: AlertHistoryItem[]
+  total: number
+}
+
+export interface Subscription {
+  id: number
+  user_id: string
+  sub_type: string
+  schedule_time?: string
+  schedule_day?: string
+  scope_type: string
+  scope_value?: string
+  enabled: boolean
+}
+
 export interface EnvStatus {
   enabled: boolean
   feeds: EnvFeedHealth[]
@@ -209,6 +237,24 @@ export async function fetchAlerts(): Promise<Alert[]> {
   return fetchJson<Alert[]>('/api/alerts/active')
 }
 
+export async function fetchAlertHistory(
+  limit: number = 50,
+  offset: number = 0,
+  type?: string,
+  severity?: string
+): Promise<AlertHistoryResponse | AlertHistoryItem[]> {
+  const params = new URLSearchParams()
+  params.set('limit', limit.toString())
+  params.set('offset', offset.toString())
+  if (type && type !== 'all') params.set('type', type)
+  if (severity && severity !== 'all') params.set('severity', severity)
+  return fetchJson<AlertHistoryResponse | AlertHistoryItem[]>(`/api/alerts/history?${params.toString()}`)
+}
+
+export async function fetchSubscriptions(): Promise<Subscription[]> {
+  return fetchJson<Subscription[]>('/api/subscriptions')
+}
+
 export async function fetchEnvStatus(): Promise<EnvStatus> {
   return fetchJson<EnvStatus>('/api/env/status')
 }