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>

artifacts/admin.py

@@ -1,6 +1,6 @@
 from django.contrib import admin
 
-from .models import Artifact, Asset, UserProfile
+from .models import ApiKey, Artifact, Asset, TagColor, UserProfile
 
 
 @admin.register(Artifact)
@@ -21,3 +21,18 @@ class AssetAdmin(admin.ModelAdmin):
     list_display = ("id", "owner", "artifact", "content_type", "size_bytes", "created_at")
     list_filter = ("owner", "content_type")
     search_fields = ("id",)
+
+
+@admin.register(ApiKey)
+class ApiKeyAdmin(admin.ModelAdmin):
+    list_display = ("name", "user", "key", "created_at", "last_used_at")
+    list_filter = ("user",)
+    search_fields = ("name", "key", "user__username")
+    # The key value and timestamps are system-managed; show but don't hand-edit.
+    readonly_fields = ("key", "created_at", "last_used_at")
+
+
+@admin.register(TagColor)
+class TagColorAdmin(admin.ModelAdmin):
+    list_display = ("name", "color")
+    search_fields = ("name",)