Refine team management: name chars, danger zone, no-team state

- Allow hyphens, @, and apostrophes in team names (backend regex)
- After delete/leave, switch to next available team instead of logging
  out; if no teams remain, show a toast prompting to create one
- Disable delete/leave button when user has only one team, with
  explanatory hint to create another team first
- Show empty state on /dashboard/team when auth has no team context,
  pointing user to the sidebar to create a team
- Fetch all teams in parallel with team detail on page load to power
  the isLastTeam guard

internal/service/team.go

@@ -18,7 +18,7 @@ import (
 	"git.omukk.dev/wrenn/sandbox/proto/hostagent/gen/hostagentv1connect"
 )
 
-var teamNameRE = regexp.MustCompile(`^[A-Za-z0-9 _]{1,128}$`)
+var teamNameRE = regexp.MustCompile(`^[A-Za-z0-9 _\-@']{1,128}$`)
 
 // TeamService provides team management operations.
 type TeamService struct {