feat: add optimistic UI for following

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[e-mail]>

src/components/organisms/grain-profile-header.js

···
     if (!this._user || this._followLoading || !this.profile) return;
 
     this._followLoading = true;
+
+    // Store previous state for rollback
+    const previousState = {
+      viewerIsFollowing: this.profile.viewerIsFollowing,
+      viewerFollowUri: this.profile.viewerFollowUri,
+      followerCount: this.profile.followerCount || 0
+    };
+
+    // Optimistic update - apply immediately
+    const newIsFollowing = !previousState.viewerIsFollowing;
+    this.profile = {
+      ...this.profile,
+      viewerIsFollowing: newIsFollowing,
+      viewerFollowUri: newIsFollowing ? this.profile.viewerFollowUri : null,
+      followerCount: previousState.followerCount + (newIsFollowing ? 1 : -1)
+    };
+
     try {
       const update = await mutations.toggleFollow(
         this.profile.handle,
         this.profile.did,
-        this.profile.viewerIsFollowing,
-        this.profile.viewerFollowUri,
-        this.profile.followerCount || 0
+        previousState.viewerIsFollowing,
+        previousState.viewerFollowUri,
+        previousState.followerCount
       );
+      // Update with real URI from server (needed for future unfollows)
       this.profile = {
         ...this.profile,
-        viewerIsFollowing: update.viewerIsFollowing,
-        viewerFollowUri: update.viewerFollowUri,
-        followerCount: update.followerCount
+        viewerFollowUri: update.viewerFollowUri
       };
     } catch (err) {
+      // Rollback on failure
       console.error('Failed to toggle follow:', err);
+      this.profile = {
+        ...this.profile,
+        viewerIsFollowing: previousState.viewerIsFollowing,
+        viewerFollowUri: previousState.viewerFollowUri,
+        followerCount: previousState.followerCount
+      };
       this.shadowRoot.querySelector('grain-toast')?.show('Failed to update');
     } finally {
       this._followLoading = false;