Visibility phase 4: no-login public viewer pages + robots

Adds the public viewing surface in the UI — shareable, no-login pages backed by
the redaction-safe /api/v1/public API:

- /p/[treeId]: tree name + searchable people directory (living people show as
  "Living person"; counts; links to person pages).
- /p/[treeId]/persons/[personId]: person detail — events, alternate names, and
  parents/partners/children as links to other public person pages.
- app/p/layout.tsx: slim public header (logo + Sign in), no app sidebar.
- robots.ts: allow /p/, disallow the authenticated app sections.
- Trees list: a "Public page ↗" link on every non-private tree so the owner can
  grab the shareable URL.

Client-rendered (same-origin fetch via Caddy). Follow-up (needs a frontend
SSR→backend base URL + a compose/env deploy step, so not auto-applied by
Watchtower): true server-rendering for SEO, a dynamic sitemap of public trees,
and per-page noindex for unlisted/site_members.

tsc clean; next build passes (both routes dynamic).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

frontend/app/trees/page.tsx

@@ -109,6 +109,17 @@ export default function TreesPage() {
                     <option value="unlisted">Unlisted</option>
                     <option value="public">Public</option>
                   </select>
+                  {tree.visibility && tree.visibility !== "private" && (
+                    <a
+                      href={`/p/${tree.id}`}
+                      target="_blank"
+                      rel="noreferrer"
+                      title="Open the public, no-login view"
+                      className="shrink-0 text-xs text-bronze hover:underline"
+                    >
+                      Public page ↗
+                    </a>
+                  )}
                   <button
                     onClick={() => remove(tree.id)}
                     className="text-[var(--muted)] hover:text-bronze"