Add avatar color, initials structure, and admin nav dropdown

- app.py: avatar_color Jinja filter using deterministic hash → lt-avatar--orange/green/purple
- base.html: proper lt-avatar--sm with lt-avatar-initials span and color class; multi-word initials support
- base.html: admin users get lt-nav-dropdown for Suppressions; non-admins see flat link; mobile drawer hides Suppressions for non-admins

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

app.py

@@ -27,6 +27,12 @@ logger = logging.getLogger('gandalf.web')
 
 app = Flask(__name__)
 
+_AVATAR_COLORS = ['lt-avatar--orange', 'lt-avatar--green', 'lt-avatar--purple', '']
+
+@app.template_filter('avatar_color')
+def avatar_color_filter(name: str) -> str:
+    return _AVATAR_COLORS[abs(hash(name)) % len(_AVATAR_COLORS)]
+
 _cfg = None