Add Dismiss label to toast button, reactivate on remind_at change

- Toast dismiss button now shows X icon + 'Dismiss' text to match
  the snooze button style
- Updating remind_at on a dismissed reminder clears is_dismissed
  and snoozed_until, making the reminder active again

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/routers/reminders.py

@@ -139,9 +139,10 @@ async def update_reminder(
 
     update_data = reminder_update.model_dump(exclude_unset=True)
 
-    # Clear stale snooze if remind_at is being changed
-    if 'remind_at' in update_data:
+    # Reactivate reminder if remind_at is being changed
+    if 'remind_at' in update_data and update_data['remind_at'] is not None:
         reminder.snoozed_until = None
+        reminder.is_dismissed = False
 
     for key, value in update_data.items():
         setattr(reminder, key, value)

frontend/src/hooks/useAlerts.tsx

@@ -114,9 +114,10 @@ export function AlertsProvider({ children }: { children: ReactNode }) {
           <button
             onClick={() => dismissRef.current(reminder.id)}
             aria-label={`Dismiss "${reminder.title}"`}
-            className="p-1 rounded hover:bg-accent/10 hover:text-accent text-muted-foreground transition-colors"
+            className="flex items-center gap-1 px-1.5 py-1 rounded hover:bg-accent/10 hover:text-accent text-muted-foreground transition-colors"
           >
             <X className="h-3.5 w-3.5" />
+            <span className="text-[11px] font-medium">Dismiss</span>
           </button>
         </div>
       </div>