refactor: made announcements time-bound and added audiences

- Added startTime and endTime properties to announcements for time-bound visibility.
- Introduced audience targeting to control who sees announcements (all, users, web_visitors).
- Updated related components and hooks to support new announcement features.
- Removed unused state tracking for dismissed announcements to streamline functionality.

surfsense_web/contracts/types/announcement.types.ts

@@ -4,12 +4,17 @@
  * Frontend-only announcement system that supports:
  * - Multiple announcement categories (update, feature, maintenance, info)
  * - Important flag for toast notifications
- * - Read/dismissed state tracking via localStorage
+ * - Time-bound visibility (start/end times)
+ * - Audience targeting (all, users, web_visitors)
+ * - Read state tracking via localStorage
  */
 
 /** Announcement category */
 export type AnnouncementCategory = "update" | "feature" | "maintenance" | "info";
 
+/** Who should see the announcement */
+export type AnnouncementAudience = "all" | "users" | "web_visitors";
+
 /** Single announcement entry */
 export interface Announcement {
 	/** Unique identifier */
@@ -22,6 +27,12 @@ export interface Announcement {
 	category: AnnouncementCategory;
 	/** ISO date string of when the announcement was published */
 	date: string;
+	/** ISO datetime — announcement becomes visible at this time */
+	startTime: string;
+	/** ISO datetime — announcement expires and is hidden after this time */
+	endTime: string;
+	/** Who should see this announcement */
+	audience: AnnouncementAudience;
 	/** If true, the user will see a toast notification for this announcement */
 	isImportant: boolean;
 	/** Optional CTA link */
@@ -37,6 +48,4 @@ export interface AnnouncementUserState {
 	readIds: string[];
 	/** IDs of important announcements already shown as toasts */
 	toastedIds: string[];
-	/** IDs of announcements the user has explicitly dismissed */
-	dismissedIds: string[];
 }