fix: improve exported add-on compatibility and better-auth handlers

[tannerlinsley]

Summary

• ignore legacy start add-on IDs when provided by exported CLI command strings
• preserve selected valid add-ons in the same command (e.g. form,shadcn,table,start,tanstack-query)
• make better-auth sign-out click handlers explicitly fire-and-forget to satisfy stricter lint/toolchain rules

Why

• some exported builder commands still include start, which is not a valid add-on id in current CLI flows and causes create to fail
• better-auth sign-out handlers can trip strict lint rules in generated apps when promise-returning handlers are not explicitly handled

Fixes #299
Fixes #301