fix: no stale stylesheet from caches (v3.6.31)

Even with the rebuilt style.css, three caching paths could still serve the
old file:

- the server sent Cache-Control: no-cache only for index.html/sw.js, so
  browsers could heuristically cache style.css;
- the SW install precached with addAll(), which re-uses HTTP-cached copies,
  so a new SW version could precache the stale file;
- the fetch handler is cache-first (fine once the cache is fresh).

Now every static shell file is served with Cache-Control: no-cache plus
Last-Modified and cheap 304 revalidation, and the SW install fetches each
precache URL with cache: 'reload' (CACHE simple-v304).  A test pins the
install behaviour.

661 frontend / 504 Python green; version 3.6.31.

Note: the public demo simple.atroshin.ru is still v3.6.20 with the old
stylesheet - it needs a deploy of the current frontend/.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v303';
+const CACHE = 'simple-v304';
 const URLS = [
   'index.html',
   'help.html',
@@ -18,8 +18,15 @@ const URLS = [
 ];
 
 self.addEventListener('install', e => {
+  // Fetch the shell with `cache: 'reload'`: a plain addAll() would re-use
+  // HTTP-cached copies, so a new SW version could precache a stale
+  // style.css/index.html (the stale-style trap, v3.6.31).
   e.waitUntil(
-    caches.open(CACHE).then(c => c.addAll(URLS))
+    caches.open(CACHE).then(c =>
+      Promise.all(URLS.map(u => fetch(u, { cache: 'reload' }).then(r => {
+        if (r && r.ok) return c.put(u, r);
+      })))
+    )
   );
   self.skipWaiting();
 });