CallPanel's hydrate body wraps the join button in a dynamic child
(`{move || if in_call {..} else {button}}`), which serializes hydration
marker comments; the SSR body rendered a *static* button (plus a
`disabled=true` the hydrate button lacks). That structural mismatch meant
`on:click` never attached on the first lobby load — the call button was dead
until a client-side nav re-rendered the subtree. Mirror the dynamic-child
wrapper and the button's serialized attributes in the SSR branch.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>