Add a study-picker discussion hub and require sign-in to view it; v2.2.0

The "Discussion" nav link previously jumped straight into the featured
study's community board. Adds /study/discussion as a proper landing page
that lists every study track so students pick which one to discuss, then
links into /study/:slug/community from there. Both pages now require an
authenticated session to view at all (previously the per-study community
page rendered its shell to logged-out visitors, only gating the post feed
itself).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "siteforge",
   "private": true,
-  "version": "2.1.1",
+  "version": "2.2.0",
   "type": "module",
   "scripts": {
     "dev": "vite",