fix: cache webmention API responses in sessionStorage to persist across refreshes

The old sessionStorage rate limiter prevented re-fetching on page refresh,
causing webmentions to disappear since they weren't in the build-time HTML.
Now caches the actual API response data with a 5-minute TTL so webmentions
render instantly from cache on refresh, while still fetching fresh data in
the background.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

js/webmentions.js

@@ -13,11 +13,6 @@
 
   if (!target || !domain) return;
 
-  // Rate limit: only fetch once per page load
-  const cacheKey = `wm-fetched-${target}`;
-  if (sessionStorage.getItem(cacheKey)) return;
-  sessionStorage.setItem(cacheKey, '1');
-
   // Use server-side proxy to keep webmention.io token secure
   // Fetch both with and without trailing slash since webmention.io
   // stores targets inconsistently (Bridgy sends different formats)
@@ -29,33 +24,47 @@
   // Check if build-time webmentions section exists
   const hasBuildTimeSection = document.getElementById('webmentions') !== null;
 
-  Promise.all([
-    fetch(apiUrl1).then((res) => res.json()).catch(() => ({ children: [] })),
-    fetch(apiUrl2).then((res) => res.json()).catch(() => ({ children: [] })),
-  ])
-    .then(([data1, data2]) => {
-      // Merge and deduplicate by wm-id
-      const seen = new Set();
-      const allChildren = [];
-      for (const wm of [...(data1.children || []), ...(data2.children || [])]) {
-        if (!seen.has(wm['wm-id'])) {
-          seen.add(wm['wm-id']);
-          allChildren.push(wm);
+  // Cache API responses in sessionStorage (5 min TTL) so webmentions
+  // persist across page refreshes without re-fetching every time
+  const cacheKey = `wm-data-${target}`;
+  const cacheTTL = 5 * 60 * 1000; // 5 minutes
+
+  function getCachedData() {
+    try {
+      const cached = sessionStorage.getItem(cacheKey);
+      if (!cached) return null;
+      const parsed = JSON.parse(cached);
+      if (Date.now() - parsed.ts > cacheTTL) {
+        sessionStorage.removeItem(cacheKey);
+        return null;
+      }
+      return parsed.children;
+    } catch {
+      return null;
     }
   }
-      const data = { children: allChildren };
-      if (!data.children || !data.children.length) return;
+
+  function setCachedData(children) {
+    try {
+      sessionStorage.setItem(cacheKey, JSON.stringify({ ts: Date.now(), children: children }));
+    } catch {
+      // sessionStorage full or unavailable - no problem
+    }
+  }
+
+  function processWebmentions(allChildren) {
+    if (!allChildren || !allChildren.length) return;
 
     let mentionsToShow;
     if (hasBuildTimeSection) {
       // Build-time section exists - only show NEW webmentions to avoid duplicates
-        mentionsToShow = data.children.filter((wm) => {
+      mentionsToShow = allChildren.filter((wm) => {
         const wmTime = new Date(wm['wm-received']).getTime();
         return wmTime > buildTime;
       });
     } else {
       // No build-time section - show ALL webmentions from API
-        mentionsToShow = data.children;
+      mentionsToShow = allChildren;
     }
 
     if (!mentionsToShow.length) return;
@@ -92,6 +101,37 @@
 
     // Update total count in main header
     updateTotalCount(mentionsToShow.length);
+  }
+
+  // Try cached data first (renders instantly on refresh)
+  const cached = getCachedData();
+  if (cached) {
+    processWebmentions(cached);
+  }
+
+  // Always fetch fresh data (updates cache for next refresh)
+  Promise.all([
+    fetch(apiUrl1).then((res) => res.json()).catch(() => ({ children: [] })),
+    fetch(apiUrl2).then((res) => res.json()).catch(() => ({ children: [] })),
+  ])
+    .then(([data1, data2]) => {
+      // Merge and deduplicate by wm-id
+      const seen = new Set();
+      const allChildren = [];
+      for (const wm of [...(data1.children || []), ...(data2.children || [])]) {
+        if (!seen.has(wm['wm-id'])) {
+          seen.add(wm['wm-id']);
+          allChildren.push(wm);
+        }
+      }
+
+      // Cache the merged results
+      setCachedData(allChildren);
+
+      // Only render if we didn't already render from cache
+      if (!cached) {
+        processWebmentions(allChildren);
+      }
     })
     .catch((err) => {
       console.debug('[Webmentions] Error fetching:', err.message);