Keep PWA builds coherent by bypassing the HTTP cache

The service worker populated a new build's cache with plain addAll(),
which the browser may satisfy from its HTTP cache — and static assets
were served without Cache-Control, so Safari's heuristic caching could
hold app.js for days. Together that could install a mixed build: a
fresh index.html whose buttons reference listeners a stale app.js never
registers.

Install now fetches assets with cache: "reload", and the server marks
all static assets no-cache (revalidation is a cheap 304; fast/offline
loads are the SW cache's job anyway).

server/main.go

@@ -806,12 +806,13 @@ func main() {
 				serveSW(w, *staticDir, swVer)
 				return
 			}
-			// PWA: manifest.json must revalidate so updates propagate.
-			if r.URL.Path == "/manifest.json" {
-				w.Header().Set("Cache-Control", "no-cache")
-			}
 			// SPA fallback: unknown paths -> index.html (so deep links work).
 			if !strings.HasPrefix(r.URL.Path, "/api/") {
+				// All static assets revalidate on every request (cheap 304s via
+				// Last-Modified). Offline/fast loads are the service worker
+				// cache's job; leaving these to the browser's heuristic HTTP
+				// caching let a stale app.js pair with a fresh index.html.
+				w.Header().Set("Cache-Control", "no-cache")
 				candidate := filepath.Join(*staticDir, filepath.FromSlash(r.URL.Path))
 				if r.URL.Path != "/" {
 					if info, err := os.Stat(candidate); err != nil || info.IsDir() {

src/index.html

@@ -277,8 +277,11 @@
           <textarea id="exercise-note" rows="5" placeholder="Reminder for how to train it, e.g. lure with a treat, mark the moment the butt touches the ground, reward"></textarea>
         </label>
         <menu>
-          <button value="delete" id="exercise-delete" class="danger" hidden>Delete</button>
-          <button value="cancel" class="ghost">Cancel</button>
+          <!-- type="button" keeps Save the form's default button, so Enter /
+               the iOS keyboard's "Go" saves instead of silently hitting the
+               (hidden) Delete button via implicit form submission. -->
+          <button type="button" value="delete" id="exercise-delete" class="danger" hidden>Delete</button>
+          <button type="button" value="cancel" class="ghost">Cancel</button>
           <button value="save" id="exercise-save">Save</button>
         </menu>
       </form>

src/sw.js

@@ -16,8 +16,14 @@ const ASSETS = [
 ];
 
 self.addEventListener("install", (event) => {
+  // cache: "reload" bypasses the browser's HTTP cache, so a new build always
+  // caches assets fetched fresh from the server. Without it, addAll could mix
+  // a fresh index.html with a heuristically-cached stale app.js and install a
+  // build whose markup references listeners the old script never registers.
   event.waitUntil(
-    caches.open(CACHE).then((cache) => cache.addAll(ASSETS))
+    caches.open(CACHE).then((cache) =>
+      cache.addAll(ASSETS.map((u) => new Request(u, { cache: "reload" })))
+    )
   );
   // No skipWaiting() here: a new worker stays in "waiting" while an old one is
   // controlling a tab, so the page can prompt before swapping assets out from