pwa: serve static assets network-first so redeploys stop serving stale JS/CSS

Asset URLs are not content-hashed, so the cache-first strategy pinned
whatever bytes were first cached and never noticed a redeployed file
changed — leaving stale weather-gl.js/output.css in place until a manual
CACHE_VERSION bump. Only a hard reload (which bypasses the SW) showed the
new assets. Switch /static/ to network-first (fall back to cache offline)
and bump CACHE_VERSION v4->v5 to flush the stale shell.

internal/web/static/sw.js

@@ -4,7 +4,7 @@
 //
 // Bump CACHE_VERSION whenever the precached shell assets change; activate()
 // drops every cache that doesn't match the current version.
-var CACHE_VERSION = "v4";
+var CACHE_VERSION = "v5";
 var SHELL_CACHE = "pete-shell-" + CACHE_VERSION;
 var RUNTIME_CACHE = "pete-runtime-" + CACHE_VERSION;
 
@@ -117,18 +117,24 @@ self.addEventListener("fetch", function (event) {
     return;
   }
 
-  // Static assets: cache-first (they're versioned by deploy), fill the cache on
-  // first miss so a later offline visit has them.
+  // Static assets: network-first. Our asset URLs are NOT content-hashed
+  // (/static/js/weather-gl.js stays the same URL across deploys), so a
+  // cache-first strategy would pin whatever bytes were first cached and never
+  // notice a redeployed file changed — leaving stale JS/CSS in place until the
+  // CACHE_VERSION bump below. Go's FileServer sets ETag/Last-Modified, so an
+  // online refetch is a cheap 304 when nothing changed. We still fill the cache
+  // on every success and fall back to it when the network is unreachable, so
+  // offline reading keeps the shell it needs.
   if (url.pathname.indexOf("/static/") === 0) {
     event.respondWith(
-      caches.match(req).then(function (hit) {
-        return hit || fetch(req).then(function (res) {
+      fetch(req).then(function (res) {
         if (res && res.ok) {
           var copy = res.clone();
           caches.open(SHELL_CACHE).then(function (cache) { cache.put(req, copy); });
         }
         return res;
-        });
+      }).catch(function () {
+        return caches.match(req);
       })
     );
     return;