fix: social activity widget - Bluesky and Mastodon feeds not rendering

- deploy.yml: add missing BLUESKY_HANDLE env var to Build site step
- deploy.yml: fix GITHUB_USERNAME secret name (was GH_USERNAME)
- mastodonFeed.js: read MASTODON_URL first, fall back to MASTODON_INSTANCE
- mastodonFeed.js: add early-return guard when instance/user vars are empty
- mastodonFeed.js: also accept MASTODON_USERNAME as fallback for MASTODON_USER

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

_data/mastodonFeed.js

@@ -6,8 +6,23 @@
 import EleventyFetch from "@11ty/eleventy-fetch";
 
 export default async function () {
-  const instance = process.env.MASTODON_INSTANCE?.replace("https://", "") || "";
-  const username = process.env.MASTODON_USER || "";
+  const instance = (
+    process.env.MASTODON_URL ||
+    process.env.MASTODON_INSTANCE ||
+    ""
+  )
+    .replace(/^https?:\/\//, "")
+    .replace(/\/+$/, "");
+  const username = (
+    process.env.MASTODON_USER ||
+    process.env.MASTODON_USERNAME ||
+    ""
+  ).trim().replace(/^@+/, "");
+
+  if (!instance || !username) {
+    console.log("[mastodonFeed] MASTODON_URL/MASTODON_USER not set, skipping");
+    return [];
+  }
 
   try {
     // First, look up the account ID