SEO panel: show the derived title and description as placeholders

[bimsonz]

The problem

The SEO panel gives no indication that its fields are optional overrides, so an empty panel reads as "this page has no SEO".

getSeoMeta already derives sensible values when the panel is empty:

// Title: SEO panel title > caller default > content title
seo.title || defaultTitle || (typeof content.data.title === "string" ? content.data.title : null)
// Description: SEO panel description > caller default > excerpt
seo.description || defaultDescription || (typeof content.data.excerpt === "string" ? content.data.excerpt : null)

Nothing in the admin surfaces that. On a site I maintain, all 21 pages have a completely blank SEO panel and every one of them emits a correct, page-specific title, description and canonical. The editor had no way to know that, and the reasonable reading of four empty boxes is that something is unset or broken. The two ways that goes wrong are retyping the page title into the SEO title, which duplicates content for no benefit and then silently goes stale when the page title changes, or assuming the site has no SEO at all.

Proposal

Show the value that will actually be used as placeholder text on SEO Title and Meta Description.

SeoPanel takes two optional props, defaultTitle and defaultDescription, matching the vocabulary getSeoMeta already uses for the same concept, and renders them as placeholders. ContentSettingsPanel passes data.title and data.excerpt, so the placeholder shows what will really be emitted rather than a generic hint. When there is nothing to derive from, no placeholder is rendered.

No change to generated meta, and no new translatable strings.

Notes

I have this implemented and passing typecheck, lint and the full admin suite locally, with two tests covering the placeholder appearing and being omitted. Happy to open the PR if this seems like a direction you would take.

One thing worth flagging: it will change the admin visual regression baselines, since the panel renders placeholder text where it previously rendered empty inputs.