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/App.vue

@@ -36,6 +36,7 @@ const events = ref([])
 const equityPoints = ref([])
 const busy = ref(false)
 const connected = ref(false)
+const socketError = ref('')
 
 let socket
 let pollTimer
@@ -132,8 +133,15 @@ onMounted(async () => {
   ])
   socket = api.openSocket({
     onMessage: handleMessage,
-    onOpen: () => (connected.value = true),
+    onOpen: () => {
+      connected.value = true
+      socketError.value = ''
+    },
     onClose: () => (connected.value = false),
+    onError: (code) => {
+      if (code === 1008) socketError.value = 'rejected by the server (token/auth)'
+      else socketError.value = `socket closed (code ${code})`
+    },
   })
   pollTimer = setInterval(refreshAll, 5000)
 })
@@ -161,6 +169,7 @@ onUnmounted(() => {
             :class="connected ? 'bg-emerald-400' : 'bg-slate-500'"
           ></span>
           {{ connected ? 'live' : 'reconnecting' }}
+          <span v-if="!connected && socketError" class="text-amber-300">· {{ socketError }}</span>
         </span>
       </div>
       <div class="text-xs text-slate-400">