Add user names, team-scoped sandbox guard, and login robustness fixes

- Add name column to users (migration + sqlc regen); propagate through JWT
  claims, auth context, all auth/OAuth handlers, service layer, and frontend
- Sidebar and team page show name instead of email; team page splits Name/Email
  into separate columns
- Block sandbox creation in UI and API when user has no active team context
- loginTeam helper falls back to first active team when no default is set,
  fixing login for invited users with no is_default membership
- Exclude soft-deleted teams from GetDefaultTeamForUser, GetBYOCTeams queries
- Guard host creation against soft-deleted teams in service/host.go
- SwitchTeam re-fetches name from DB instead of trusting stale JWT claim
- Reset teams store on login so stale data from a previous session never persists
- Update openapi.yaml: add name to SignupRequest and AuthResponse schemas

frontend/src/lib/components/Sidebar.svelte

@@ -27,7 +27,7 @@
 	let teamPopoverOpen = $state(false);
 
 	let currentTeamName = $derived(teamsStore.list.find((t) => t.id === auth.teamId)?.name ?? '');
-	let userName = $derived(auth.email ?? '');
+	let userName = $derived(auth.name || auth.email || '');
 
 	// Create team dialog
 	let showCreateTeam = $state(false);