Shared editor language selection; paced WebSocket reconnects

- LangSelect.vue: one language selector for the page and structure tabs
  (small flag, clean dropdown), v-modeled on the shell-wide editorLang —
  switching in either panel switches both AND the page preview. While the
  panel is open the selection overrides the normal language preferences
  (swapdoc.setLangOverride: loadPlain and pagerite.js fetches/prefetches
  pin ?lang=, the primary language by its own code); closing restores.
- reconnect.js: every socket (page/banner editors, analytics view, the
  activity channel) now connects through a staggered slot — simultaneous
  attempts at page load (Vite's HMR socket plus ours, repeated on every
  refresh) trip the browser's WebSocket throttling, leaving all sockets
  to the host "pending" for minutes, which was the recurring empty
  editor. A watchdog closes sockets stuck CONNECTING so they reschedule
  through the policy (jittered exponential backoff, reset only by a
  healthy connection) instead of hanging forever; a reconnected editor
  re-opens its document when the previous open died with its socket.

frontend/src/reconnect.js

@@ -0,0 +1,56 @@
+// Shared reconnect policy for the WebSockets (page/banner editors,
+// analytics view, the activity channel). Two things trip a browser's
+// WebSocket throttling, after which every socket to the host sits
+// "pending" (never opens, never closes) for minutes:
+//
+//  1. A burst of simultaneous attempts — page load opens Vite's HMR
+//     socket plus several of ours at the same moment, and every refresh
+//     repeats the burst. socketSlot() spaces new sockets out.
+//  2. Too-frequent retries — so failed attempts back off exponentially
+//     (a few seconds, doubling to half a minute), reset only after a
+//     connection stayed open long enough to count as healthy. A socket
+//     that closes right after opening must NOT reset the backoff.
+export function reconnectPolicy({ min = 2000, max = 30000, healthyAfter = 30000 } = {}) {
+  let delay = min
+  let openedAt = 0
+  return {
+    // Stamp a socket that just opened.
+    opened() {
+      openedAt = Date.now()
+    },
+    // The socket closed: the wait before the next attempt (up to 50%
+    // jitter; the base doubles per failure). A healthy streak resets it.
+    closed() {
+      if (openedAt && Date.now() - openedAt >= healthyAfter) delay = min
+      openedAt = 0
+      const wait = Math.round(delay * (1 + Math.random() * 0.5))
+      delay = Math.min(delay * 2, max)
+      return wait
+    },
+  }
+}
+
+// Sockets created at the same moment (page load: Vite's HMR socket plus
+// ours) read as one burst to the browser's throttling. Space new sockets
+// out: each call reserves a slot a beat after the previous one.
+let nextSlot = 0
+export function socketSlot() {
+  const now = Date.now()
+  const wait = Math.max(0, nextSlot - now)
+  nextSlot = Math.max(now, nextSlot) + 300
+  return wait
+}
+
+// A socket still CONNECTING after this long counts as a failed attempt:
+// browser throttling leaves sockets "pending" (no open, no close) for
+// minutes, and without a watchdog the app would wait on one forever (the
+// recurring empty editor). Closing it fires onclose, which reschedules
+// through the policy's backoff — it never reconnects aggressively itself.
+export function watchConnecting(ws, label) {
+  return setTimeout(() => {
+    if (ws.readyState === WebSocket.CONNECTING) {
+      console.warn(`[pagerite] ${label} socket stuck connecting — closing it, retrying with backoff`)
+      ws.close()
+    }
+  }, 10_000)
+}