v0.2.1: top-bar avatar, You/Admin settings groups, admin models

- Replace the gear/username button with a round initials avatar (per-user
  deterministic color via a new daybook_extras template filter); dropdown
  now shows the username and keeps Settings/About/theme/logout. Sets up an
  optional profile photo later.
- Regroup the settings nav into "You" (Account, API Keys, Storage, Demo
  Data) and a superuser-only "Admin" (Users), About separate — the split
  now reflects who each setting affects, not a flat list.
- Register ApiKey (key read-only) and TagColor in Django admin so /admin/
  is complete as the operator break-glass tool, distinct from the themed
  Settings product surface.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

static/daybook.css

@@ -128,6 +128,42 @@ body {
   border-color: var(--control-border);
 }
 
+.avatar-button {
+  background: transparent;
+  border: 0;
+  border-radius: 50%;
+  cursor: pointer;
+  line-height: 0;
+  padding: 2px;
+}
+
+.avatar-button:hover .avatar,
+.avatar-button:focus-visible .avatar {
+  box-shadow: 0 0 0 2px var(--content-bg), 0 0 0 4px var(--control-border);
+}
+
+.avatar {
+  align-items: center;
+  border-radius: 50%;
+  color: #fff;
+  display: inline-flex;
+  font-size: 0.9rem;
+  font-weight: 600;
+  height: 2rem;
+  justify-content: center;
+  line-height: 1;
+  width: 2rem;
+}
+
+.user-menu-name {
+  border-bottom: 1px solid var(--control-border);
+  color: var(--muted);
+  font-size: 0.8rem;
+  font-weight: 600;
+  margin-bottom: 0.25rem;
+  padding: 0.5rem 0.8rem;
+}
+
 .user-menu-dropdown {
   background: var(--surface);
   border: 1px solid var(--control-border);
@@ -968,6 +1004,21 @@ kbd {
   padding-bottom: 0.7rem !important;
 }
 
+.settings-nav-group {
+  display: flex;
+  flex-direction: column;
+  margin-bottom: 0.9rem;
+}
+
+.settings-nav-heading {
+  color: var(--muted);
+  font-size: 0.7rem;
+  font-weight: 700;
+  letter-spacing: 0.06em;
+  padding: 0.3rem 0.7rem;
+  text-transform: uppercase;
+}
+
 .settings-panel {
   max-width: 42rem;
   padding: 1.6rem 2rem;