feat(frontend): Service Worker WS, mode édition dashboard, sidebar click-to-toggle

- WebSocket via Service Worker (ws.sw.js) : connexions persistantes entre navigations
  Swup, reconnexion exponentielle, protocole WS_SUBSCRIBE/WS_UNSUBSCRIBE/WS_SEND
- WsProxy dans app.js : abstraction SW + fallback WebSocket direct
- proxmoxPage migré vers WsProxy (identique au dashboardPage)
- Dashboard : mode édition toggle — DnD, resize (x1/x2), masquer/afficher widget
  uniquement actifs en mode édition ; preview drag (is-dragging/drag-over)
- Sidebar : suppression bouton hamburger, clic sur sidebar-header pour replier
- pages.css : targets-grid 350px, styles edit mode, widget-size-2, drag preview
- neu.css : sidebar-header cursor pointer, suppression .sidebar-toggle

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

frontend/js/ws.sw.js

@@ -0,0 +1,86 @@
+/**
+ * ProxmoxPanel — WebSocket Service Worker
+ * Maintient les connexions WS en vie entre les navigations Swup.
+ * Les pages s'abonnent/désabonnent via postMessage.
+ */
+'use strict'
+
+// channel → { ws: WebSocket|null, url: string, clientIds: Set<string>, retryDelay: number }
+const connections = new Map()
+
+self.addEventListener('install', () => self.skipWaiting())
+
+self.addEventListener('activate', e =>
+  e.waitUntil(self.clients.claim())
+)
+
+self.addEventListener('message', event => {
+  const { type, channel, url, payload } = event.data || {}
+  if (!channel) return
+  const clientId = event.source?.id
+  if (!clientId) return
+
+  switch (type) {
+    case 'WS_SUBSCRIBE': {
+      let conn = connections.get(channel)
+      if (!conn) {
+        conn = { ws: null, url: null, clientIds: new Set(), retryDelay: 2000 }
+        connections.set(channel, conn)
+      }
+      conn.clientIds.add(clientId)
+      if (url) conn.url = url
+      ensureWS(channel)
+      break
+    }
+    case 'WS_UNSUBSCRIBE': {
+      const conn = connections.get(channel)
+      if (conn) conn.clientIds.delete(clientId)
+      break
+    }
+    case 'WS_SEND': {
+      const conn = connections.get(channel)
+      if (conn?.ws?.readyState === 1) conn.ws.send(payload)
+      break
+    }
+  }
+})
+
+function ensureWS(channel) {
+  const conn = connections.get(channel)
+  if (!conn?.url) return
+  if (conn.ws && conn.ws.readyState < 2) return // CONNECTING (0) ou OPEN (1)
+
+  const ws = new WebSocket(conn.url)
+  conn.ws = ws
+  conn.retryDelay = 2000
+
+  notify(channel, 'WS_STATUS', { status: 'connecting' })
+
+  ws.onopen = () => {
+    conn.retryDelay = 2000
+    notify(channel, 'WS_STATUS', { status: 'connected' })
+  }
+
+  ws.onmessage = e => notify(channel, 'WS_MESSAGE', { data: e.data })
+
+  ws.onerror = () => notify(channel, 'WS_STATUS', { status: 'error' })
+
+  ws.onclose = () => {
+    notify(channel, 'WS_STATUS', { status: 'disconnected' })
+    // Backoff exponentiel plafonné à 30s
+    const delay = conn.retryDelay
+    conn.retryDelay = Math.min(conn.retryDelay * 1.5, 30000)
+    setTimeout(() => ensureWS(channel), delay)
+  }
+}
+
+async function notify(channel, type, extra) {
+  const conn = connections.get(channel)
+  if (!conn || conn.clientIds.size === 0) return
+  const allClients = await self.clients.matchAll({ type: 'window' })
+  for (const client of allClients) {
+    if (conn.clientIds.has(client.id)) {
+      client.postMessage({ channel, type, ...extra })
+    }
+  }
+}