fix(admin): allow owner and admin account roles in user creation and role management

Four places were hardcoded to engineer|viewer only:
- AccountRoleUpdate schema (user.py) — blocked PUT /admin/users/{id}/account-role at the API level
- AdminUserCreate schema (admin.py) — blocked creating users with owner/admin role
- AccountDetailPage role dropdowns (create form + inline member role changer)
- AccountsPage create user role dropdown

Now all four accept the full set: owner, admin, engineer, viewer.

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

backend/app/schemas/admin.py

@@ -319,7 +319,7 @@ class AdminUserCreate(BaseModel):
     name: str = Field(..., min_length=1, max_length=255)
     account_mode: Literal["existing", "personal"]
     account_display_code: Optional[str] = Field(None, description="Required when account_mode='existing'")
-    account_role: Optional[Literal["engineer", "viewer"]] = Field(None, description="Required when account_mode='existing'")
+    account_role: Optional[Literal["owner", "admin", "engineer", "viewer"]] = Field(None, description="Required when account_mode='existing'")
     send_email: bool = True