Public view: add tree chart + homepage Explore links

Two gaps from review of the public surface:
- The public tree page showed only a list of names. Add the family-chart
  hourglass (PublicTreeChart) above the directory — the same renderer the
  member tree view uses, including the cycle-sanitisation that guards against a
  bad graph, fed by redacted public data. Click a card to recenter; "Open"
  links to the person's public page. Centers on the tree's home person.
- The homepage had no path to /explore. Add an "Explore" nav link and an
  "Explore public trees" hero button.

tsc clean; next build passes.

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

frontend/app/page.tsx

@@ -31,6 +31,9 @@ export default function Home() {
             <img src="/provenance-logo-plain.svg" alt="Provenance" className="h-7 w-auto" />
           </Link>
           <nav className="flex items-center gap-5 text-sm">
+            <Link href="/explore" className="text-[var(--muted)] hover:text-[var(--foreground)]">
+              Explore
+            </Link>
             <Link href="/trees" className="text-[var(--muted)] hover:text-[var(--foreground)]">
               Trees
             </Link>
@@ -66,6 +69,11 @@ export default function Home() {
                   Sign in
                 </Button>
               </Link>
+              <Link href="/explore">
+                <Button size="lg" variant="ghost">
+                  Explore public trees
+                </Button>
+              </Link>
             </div>
           </div>