Stop caching index.html and speed up socket reconnect

Phones offer no easy hard-refresh, so a cached index.html kept serving an old
bundle after a deploy (the live feed showed "reconnecting"). index.html and the
SPA catch-all now send Cache-Control: no-store, while hashed /assets stay
immutable. Also retry the first socket reconnect after 1s (was 2s) and surface
the close code in the header so a failure is actionable on a phone.

frontend/src/api.js

@@ -26,7 +26,7 @@ export const getPresets = () => http.get('/presets').then((r) => r.data)
 export const createPreset = (payload) => http.post('/presets', payload).then((r) => r.data)
 export const deletePreset = (id) => http.delete(`/presets/${id}`).then((r) => r.data)
 
-export function openSocket({ onMessage, onOpen, onClose }) {
+export function openSocket({ onMessage, onOpen, onClose, onError }) {
   const proto = window.location.protocol === 'https:' ? 'wss' : 'ws'
   const url = `${proto}://${window.location.host}/api/ws`
   let socket
@@ -36,9 +36,9 @@ export function openSocket({ onMessage, onOpen, onClose }) {
 
   const schedule = () => {
     if (closed) return
-    // Back off from 2s up to 30s so a persistently failing socket (e.g. auth)
-    // does not hammer the server, while a brief blip still reconnects fast.
-    const delay = Math.min(2000 * 2 ** attempt, 30000)
+    // Retry at 1s, then back off up to 30s so a persistently failing socket
+    // does not hammer the server while a brief blip recovers immediately.
+    const delay = Math.min(1000 * 2 ** attempt, 30000)
     attempt += 1
     retryTimer = setTimeout(connect, delay)
   }
@@ -51,8 +51,11 @@ export function openSocket({ onMessage, onOpen, onClose }) {
       onOpen?.()
     }
     socket.onmessage = (event) => onMessage?.(JSON.parse(event.data))
-    socket.onclose = () => {
+    socket.onclose = (event) => {
       onClose?.()
+      // 1008 = policy violation (token rejected); 1006 = abnormal close, which
+      // is what an auth/proxy rejection looks like to the browser.
+      onError?.(event.code, event.reason)
       schedule()
     }
     socket.onerror = () => socket.close()