feat: legal (Impressum), contact, page template, and map shortcode

- page.html generic single-page template
- legal.md with slug=impressum to match live URL
- contact.md with practice info, hours, map embed
- map shortcode (iframe-based) usable from markdown
- Home template inlines the map iframe directly (shortcodes can't be called from templates)

templates/shortcodes/map.html

@@ -0,0 +1,11 @@
+<section class="map-section">
+  <iframe
+    src="https://maps.google.com/maps?q=Kaiserstr.+23,+42329+Wuppertal&t=&z=15&ie=UTF8&iwloc=&output=embed"
+    width="100%"
+    height="450"
+    style="border:0"
+    loading="lazy"
+    referrerpolicy="no-referrer-when-downgrade"
+    title="Karte: Praxis am Lienhardplatz">
+  </iframe>
+</section>