fix(auth): align CLI callback page colors with web app theme

Update the local OAuth callback HTML to use the hotdata app custom
gray palette (#121212 bg, #1a1a1a card, #333333 border) instead of
default Tailwind grays, so the post-login page matches the web UI.

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

Author: eddietejeda

src/auth.rs

@@ -207,16 +207,16 @@ fn receive_callback(
     *, *::before, *::after {{ box-sizing: border-box; margin: 0; padding: 0; }}
     body {{
       font-family: ui-sans-serif, system-ui, -apple-system, sans-serif;
-      background: #111827;
+      background: #121212;
       color: #e5e7eb;
       display: flex;
       align-items: center;
       justify-content: center;
       min-height: 100vh;
     }}
     .card {{
-      background: #1f2937;
-      border: 1px solid #374151;
+      background: #1a1a1a;
+      border: 1px solid #333333;
       border-radius: 0.5rem;
       padding: 2.5rem;
       max-width: 420px;