Use page_title assign in <.header>

I’m not sure if it would be better for phx.gen.html or phx.gen.live to use the @page_title assign by default in <.header>, but doing so would ensure the page title and header stay in sync—since you’d only need to set it in one place.

      <.header>
        {@page_title}
        <:actions>
          <.button variant="primary" navigate={~p"/posts/new"}>
            <.icon name="hero-plus" /> New post
          </.button>
        </:actions>
      </.header>

This applies to index.html.heex and show.html.heex, since form.html.heex already follows that pattern.

I’d be happy to make those changes if you agree and find it useful.

I don’t have strong opinions here. Currently, phx.gen.html doesn’t set a page_title so if we do this, we should probably change that so it is consistent.