Fix WebSocket auth flow and improve header user profile display

- Fix NIP-42 auth race condition: wait for AUTH challenge before authenticating
- Header user profile: avatar fills vertical space, username vertically centered
- Remove username truncation to show full name/npub
- Standardize header height to 3em across all components

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/web/src/Sidebar.svelte

@@ -47,7 +47,7 @@
     .sidebar {
         position: fixed;
         left: 0;
-        top: 2.5em;
+        top: 3em;
         width: 200px;
         bottom: 0;
         background: var(--sidebar-bg);