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/index.html

@@ -1713,7 +1713,7 @@
 // ===== Version =====
 // Single source of truth for the PWA version: shown in the header and used
 // by the server version check in pysimConnect().
-const SIMPLE_VERSION = '3.6.30';
+const SIMPLE_VERSION = '3.6.31';
 document.getElementById('app-version').textContent = 'v' + SIMPLE_VERSION;
 
 // ===== Tab switching =====

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();
 });

frontend/tests/sw.test.js

@@ -55,6 +55,23 @@ function navigateEvent(url) {
 	return event;
 }
 
+test('install precaches every URL with cache: reload (no stale HTTP copies)', async () => {
+	const calls = [];
+	const { listeners, puts } = loadSW({
+		fetchImpl: async (url, init) => {
+			calls.push([String(url), init && init.cache]);
+			return { ok: true, status: 200 };
+		},
+		cacheMatch: async () => undefined,
+	});
+	const event = { waitUntil: p => { event.promise = p; } };
+	listeners.install(event);
+	await event.promise;
+	assert.ok(calls.length >= 10, String(calls.length));
+	assert.ok(calls.every(c => c[1] === 'reload'), JSON.stringify(calls[0]));
+	assert.strictEqual(puts.length, calls.length);
+});
+
 test('offline navigation falls back to the cached index.html', async () => {
 	const index = new FakeResponse('html');
 	const { listeners } = loadSW({