iPX opens and reads without a connection (#389)

With no network the home-screen app showed nothing: no service worker,
and the lists the page had fetched, full text included, lived only in
its memory.

A service worker (web/sw/sw.ts, TypeScript checked against a worker's
globals by tsconfig.sw.json, built by swc to /sw.js) goes to the network
first for the page and every API read and keeps the answers, so online
nothing is an old copy and #388's version check works as before;
offline the kept copy answers. Scripts named by hash come from what is
kept. Reads, flags and places in an episode that cannot reach the
server wait in an outbox, sent in order when the connection is back
(and new ones queue behind them meanwhile, so none lands out of turn);
a kept list served offline shows the reads waiting there.

Pictures: a worker cannot keep another site's pictures sensibly, since
browsers count each opaque answer as megabytes. ipx now remembers the
picture addresses in item text it sends (bounded, in memory) and
/api/art fetches those as it does artwork, through the guarded client;
the worker keeps them, for unread items, under a 200 MB budget, and
serves them offline in place of the publisher's. Online, pictures still
come from their publisher.

Sign-out, or another account signing in on the device, clears what was
kept. The browser suite blocks the worker except in the offline tests,
since it answers before page.route can.

The fixture JPEGs are placeholders a browser cannot draw; the offline
test uses a real PNG.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/dialogs.ts

@@ -846,9 +846,11 @@ $<HTMLButtonElement>('#scanAll').onclick=scanAll;
 $<HTMLButtonElement>('#prefs').onclick=prefsModal;
 // Someone the proxy signed in is signed out by the proxy: ipx's own sign-out cannot stick while
 // the proxy still vouches for them. /api/me says where, when that is the case.
-$<HTMLButtonElement>('#signout').onclick=async()=>{ await api('/api/logout',{method:'POST'}); location.href=S.me?.sign_out||'/login'; };
+// What was kept for offline goes too: the next person on this device is not to read it (#389).
+$<HTMLButtonElement>('#signout').onclick=async()=>{ await api('/api/logout',{method:'POST'}); await clearOffline(); location.href=S.me?.sign_out||'/login'; };
 api('/api/me').then(u=>{
   S.me=u;
+  offlineOwner(u.name);
   $('#who').textContent=u.name+(u.admin?' · admin':'');
   // ipx's version and the page's, each released on its own.
   // Also at the foot of Settings: the tooltip alone was out of reach of a keyboard and of a phone (#309).