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/pages/ChatShellPage.tsx

@@ -1,11 +1,13 @@
 import { useCallback, useEffect, useState } from 'react'
 import { useNavigate, useParams } from 'react-router-dom'
+import { NetworkError } from '../api/client'
 import { listMyInvites } from '../api/invites'
 import { listMyRooms, listRoomMembers } from '../api/rooms'
 import { BrowseRoomsModal } from '../components/BrowseRoomsModal'
 import { ChatPane } from '../components/ChatPane'
 import { InvitesModal } from '../components/InvitesModal'
 import { NewRoomModal } from '../components/NewRoomModal'
+import { OfflineBanner } from '../components/OfflineBanner'
 import { RoomInfoPanel } from '../components/RoomInfoPanel'
 import { Sidebar } from '../components/Sidebar'
 import { TopBar } from '../components/TopBar'
@@ -27,13 +29,23 @@ export function ChatShellPage() {
   const [inviteCount, setInviteCount] = useState(0)
   const [infoOpen, setInfoOpen] = useState(false)
   const [modal, setModal] = useState<ModalKind>(null)
+  const [roomsUnavailableOffline, setRoomsUnavailableOffline] = useState(false)
 
   const activeRoom = rooms.find((r) => r.id === roomId)
 
   const refreshRooms = useCallback(async () => {
-    const list = await listMyRooms()
-    setRooms(list)
-    return list
+    try {
+      const list = await listMyRooms()
+      setRooms(list)
+      setRoomsUnavailableOffline(false)
+      return list
+    } catch (err) {
+      if (err instanceof NetworkError) {
+        setRoomsUnavailableOffline(true)
+        return []
+      }
+      throw err
+    }
   }, [])
 
   const refreshMembers = useCallback(() => {
@@ -62,6 +74,7 @@ export function ChatShellPage() {
   return (
     <div className="chat-shell">
       <TopBar />
+      <OfflineBanner />
       <div className="chat-shell-body">
         {(!isMobile || !roomId) && (
           <Sidebar
@@ -73,6 +86,7 @@ export function ChatShellPage() {
             onOpenBrowse={() => setModal('browse')}
             onOpenInvites={() => setModal('invites')}
             inviteCount={inviteCount}
+            unavailableOffline={roomsUnavailableOffline && rooms.length === 0}
           />
         )}