Phase 3: PWA offline caching (frontend only)

Adds a real Workbox runtime-caching strategy on top of the Phase 1 app-shell
precache: StaleWhileRevalidate (cache-and-refresh) for the five read
endpoints (rooms/mine, open rooms, room messages, room members, invites/mine)
with a bounded/expiring cache per endpoint, while /api/auth/* and all
mutations stay network-only. An OfflineBanner (navigator.onLine-driven) and
a clearer Composer status line ("Connecting..." vs "You're offline") surface
what's actually happening; api/client.ts gains a NetworkError distinct from
ApiError so a genuine cache-miss-while-offline shows a quiet empty state
instead of a red error.

Manual offline testing (backend stopped, `vite preview` against the real
production service worker) surfaced a real gap the plan hadn't accounted
for: GET /api/auth/me is intentionally NetworkOnly, but that meant
ProtectedRoute could never confirm a session while offline and always
bounced to /login -- none of the newly-cached room/message data was ever
reachable. Fixed by caching a minimal, non-sensitive "last known user" in
localStorage (lib/lastUser.ts) and having AuthContext fall back to it for
any *unconfirmed* auth check (network failure, or a down backend answering
through a live reverse proxy with its own 502/503/504 -- both happen in
real deployments, not just literal airplane-mode). Only a server-confirmed
401 still clears it and signs the user out; every real action still
re-checks the actual session cookie server-side, so this can't grant
anything -- it only keeps cached UI reachable.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/ChatPane.tsx

@@ -1,5 +1,6 @@
 import { useCallback, useEffect, useState } from 'react'
 import { useNavigate } from 'react-router-dom'
+import { NetworkError } from '../api/client'
 import { getRoomMessages } from '../api/rooms'
 import { useChatSocket } from '../ws/useChatSocket'
 import type { ChatMessageEnvelope, Message, MyRoomItem, RoomMember, ServerEnvelope } from '../types'
@@ -21,12 +22,22 @@ export function ChatPane({ room, members, isMobile, onBack, onToggleInfo, infoOp
   const [history, setHistory] = useState<Message[]>([])
   const [live, setLive] = useState<ChatMessageEnvelope[]>([])
   const [wsError, setWsError] = useState<string | null>(null)
+  const [historyUnavailableOffline, setHistoryUnavailableOffline] = useState(false)
 
   useEffect(() => {
     setHistory([])
     setLive([])
     setWsError(null)
-    getRoomMessages(room.id).then(setHistory).catch((err) => setWsError(String(err)))
+    setHistoryUnavailableOffline(false)
+    getRoomMessages(room.id)
+      .then(setHistory)
+      .catch((err) => {
+        if (err instanceof NetworkError) {
+          setHistoryUnavailableOffline(true)
+        } else {
+          setWsError(String(err))
+        }
+      })
   }, [room.id])
 
   const onMessage = useCallback((envelope: ServerEnvelope) => {
@@ -71,6 +82,11 @@ export function ChatPane({ room, members, isMobile, onBack, onToggleInfo, infoOp
       </header>
 
       {wsError && <p className="chat-pane-error">{wsError}</p>}
+      {historyUnavailableOffline && (
+        <p className="chat-pane-error chat-pane-note">
+          Message history for this room isn't available offline yet.
+        </p>
+      )}
 
       <MessageList messages={[...history, ...live]} members={members} />
       <Composer roomName={room.name} disabled={!connected} onSend={send} />