feat: multi-domain fediverse support and share-to-mastodon upgrade

- Replace single localStorage string with versioned multi-domain store
  (fediverse_domains_v1) with usage tracking, inspired by Mastodon's
  share.joinmastodon.org project
- Add domain validation via URL constructor before redirecting
- Add mode param to fediverseInteract component: "interact" for
  authorize_interaction, "share" for /share?text=...
- Migrate old fediverse_instance key automatically on first load
- Extract shared modal partial (fediverse-modal.njk) used by post
  interaction, follow widget, and share widget
- Share widget now prompts visitors for their own instance instead of
  hardcoding site owner's Mastodon instance

Confab-Link: http://localhost:8080/sessions/0ec83454-d346-4329-8aaf-6b12139bf596

js/fediverse-interact.js

@@ -1,31 +1,102 @@
 /**
- * Fediverse remote interaction component (Alpine.js)
- * Enables users to like/boost/reply from their own fediverse instance
- * via the authorize_interaction endpoint.
+ * Fediverse interaction & sharing component (Alpine.js)
  *
+ * Two modes:
+ *   - "interact" (default): redirect to authorize_interaction for like/boost/reply/follow
+ *   - "share": redirect to /share?text=... for composing a new post
+ *
+ * Stores multiple domains in localStorage with usage tracking.
  * Registered via Alpine.data() so the component is available
  * regardless of script loading order.
  */
 
+const STORAGE_KEY = "fediverse_domains_v1";
+const OLD_STORAGE_KEY = "fediverse_instance";
+
+function loadDomains() {
+  try {
+    const json = localStorage.getItem(STORAGE_KEY);
+    if (json) return JSON.parse(json);
+  } catch { /* corrupted data */ }
+
+  // Migrate from old single-domain key
+  const old = localStorage.getItem(OLD_STORAGE_KEY);
+  if (old) {
+    const domains = [{ domain: old, used: 1, lastUsed: new Date().toISOString() }];
+    localStorage.setItem(STORAGE_KEY, JSON.stringify(domains));
+    localStorage.removeItem(OLD_STORAGE_KEY);
+    return domains;
+  }
+
+  return [];
+}
+
+function saveDomains(domains) {
+  localStorage.setItem(STORAGE_KEY, JSON.stringify(domains));
+}
+
+function addDomain(domain) {
+  const domains = loadDomains();
+  const existing = domains.find((d) => d.domain === domain);
+  if (existing) {
+    existing.used += 1;
+    existing.lastUsed = new Date().toISOString();
+  } else {
+    domains.push({ domain, used: 1, lastUsed: new Date().toISOString() });
+  }
+  saveDomains(domains);
+  return domains;
+}
+
+function removeDomain(domain) {
+  const domains = loadDomains().filter((d) => d.domain !== domain);
+  saveDomains(domains);
+  return domains;
+}
+
+function isValidDomain(str) {
+  try {
+    return new URL(`https://${str}`).hostname === str;
+  } catch {
+    return false;
+  }
+}
+
 document.addEventListener("alpine:init", () => {
-  Alpine.data("fediverseInteract", (postUrl) => ({
-    postUrl,
+  Alpine.data("fediverseInteract", (targetUrl, mode) => ({
+    targetUrl,
+    mode: mode || "interact",
     showModal: false,
     instance: "",
+    savedDomains: [],
+    showInput: false,
+    error: "",
 
     handleClick(event) {
       event.preventDefault();
-      const saved = localStorage.getItem("fediverse_instance");
-      if (saved && !event.shiftKey) {
-        this.redirectToInstance(saved);
+      this.savedDomains = loadDomains().sort((a, b) => b.used - a.used);
+
+      if (this.savedDomains.length === 1 && !event.shiftKey) {
+        addDomain(this.savedDomains[0].domain);
+        this.redirectToInstance(this.savedDomains[0].domain);
         return;
       }
-      this.openModal(saved);
+
+      if (this.savedDomains.length === 0) {
+        this.showInput = true;
+      } else {
+        this.showInput = false;
+      }
+
+      this.instance = "";
+      this.error = "";
+      this.showModal = true;
     },
 
-    openModal(prefill) {
-      this.instance = prefill || "";
-      this.showModal = true;
+    showAddNew() {
+      this.showInput = true;
+      this.instance = "";
+      this.error = "";
       this.$nextTick(() => {
         const input = this.$refs.instanceInput;
         if (input) input.focus();
@@ -37,14 +108,37 @@ document.addEventListener("alpine:init", () => {
       if (!domain) return;
       // Strip protocol and trailing slashes
       domain = domain.replace(/^https?:\/\//, "").replace(/\/+$/, "");
-      localStorage.setItem("fediverse_instance", domain);
+
+      if (!isValidDomain(domain)) {
+        this.error = "Please enter a valid domain (e.g. mastodon.social)";
+        return;
+      }
+
+      this.error = "";
+      this.savedDomains = addDomain(domain);
       this.showModal = false;
       this.redirectToInstance(domain);
     },
 
+    useSaved(domain) {
+      this.savedDomains = addDomain(domain);
+      this.showModal = false;
+      this.redirectToInstance(domain);
+    },
+
+    deleteSaved(domain) {
+      this.savedDomains = removeDomain(domain);
+      if (this.savedDomains.length === 0) {
+        this.showInput = true;
+      }
+    },
+
     redirectToInstance(domain) {
-      const url = `https://${domain}/authorize_interaction?uri=${encodeURIComponent(this.postUrl)}`;
-      window.location.href = url;
+      if (this.mode === "share") {
+        window.location.href = `https://${domain}/share?text=${encodeURIComponent(this.targetUrl)}`;
+      } else {
+        window.location.href = `https://${domain}/authorize_interaction?uri=${encodeURIComponent(this.targetUrl)}`;
+      }
     },
   }));
 });