feat(phase3): PWA, auto-discovery, bookmarklet, multi-tab sync

- PWA: manifest, service worker (cache-first static, network-first API),
  offline page, install prompt banner
- Auto-discovery: Docker socket + Traefik API scanning, approval UI
- Quick-add bookmarklet: popup-based add page, favicon auto-detect
- Multi-tab sync: BroadcastChannel for theme + data changes
- i18n translations for all new strings (EN/RU)

PLAN_PROMPT.md

@@ -448,7 +448,7 @@ To avoid scope creep, the MVP should include:
 - Additional widget types
 
 ### Phase 3
-- Auto-discovery (Docker/Traefik)
+- ~~Auto-discovery (Docker/Traefik)~~ **DONE** — Phase 5 implementation: discoveryService.ts, /api/admin/discover endpoints, DiscoveryPanel.svelte, SettingsForm discovery config, i18n EN/RU
 - Import/Export
 - PWA
 - Ping history sparklines