feat(brand): switch user-facing copy + Mercure topic prefix to wevisto.com

Mercure topic identifiers updated in lockstep across PHP publisher + TS
subscriber (and their tests). Help-page setup instructions now point to
wevisto.com. Traefik already serves both hosts; this aligns the in-app
references with the public brand.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/composables/useDeviceMercure.ts

@@ -3,7 +3,7 @@ import { storeToRefs } from 'pinia'
 import { useDevicesStore } from '@/stores/devices'
 import type { Device } from '@/types'
 
-const TOPIC_PREFIX = 'https://pictureframe.edholm.me/devices/'
+const TOPIC_PREFIX = 'https://wevisto.com/devices/'
 
 declare global {
   interface Window {