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 =====