Global Import in the menu; mobile drawer nav

- Add a top-level "Import" entry to the sidebar and a global /import page, so
  you can start a tree from a GEDCOM without first creating an empty one. The
  import flow now picks its destination (new tree, or an existing one) — the
  tree-scoped page reuses the same <GedcomImport> with a fixed destination and
  keeps Export.
- Extract the sidebar chrome into <AppShell> and give small screens a working
  menu: a hamburger opens the full sidebar as a slide-in drawer (it was just a
  logo + "Trees" link before). Used by both /trees and /import.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/app/import/layout.tsx

@@ -0,0 +1,5 @@
+import { AppShell } from "@/components/app-shell";
+
+export default function ImportLayout({ children }: { children: React.ReactNode }) {
+  return <AppShell>{children}</AppShell>;
+}

frontend/app/import/page.tsx

@@ -0,0 +1,15 @@
+import { GedcomImport } from "@/components/gedcom-import";
+
+export default function ImportPage() {
+  return (
+    <div className="space-y-6">
+      <div>
+        <h1 className="text-2xl font-semibold">Import</h1>
+        <p className="mt-1 text-sm text-[var(--muted)]">
+          Bring in a GEDCOM file — start a brand-new tree, or add to one you already have.
+        </p>
+      </div>
+      <GedcomImport />
+    </div>
+  );
+}