chore(social-client): drop redundant comment, harden defineProperty

Per code-review:
- Drop the inline comment in sendFriendRequest; the commit message
  for the prior commit already covers the why and CLAUDE.md prefers
  no comments when the code is self-explanatory.
- Add writable: true to the window.location defineProperty in the
  test so re-firing beforeEach across jsdom version drift is safe.

packages/web/src/stores/socialStore.ts

@@ -218,7 +218,6 @@ export const useSocialStore = create<SocialState>((set, get) => ({
         res = await api.social.sendRequest(username);
       } else {
         const baseName = username.slice(0, atIndex);
-        // Hostnames are case-insensitive; comparison RHS is canonical lowercase.
         const domain = username.slice(atIndex + 1).toLowerCase();
 
         // Check if domain matches home instance