fix zola.toml config table scoping bug and JSON-LD safety

[[extra.menu]] silently swallowed ~20 later extra.* keys (posts_page_title,
search_placeholder, show_rss_link, notfound_*, etc.) into the last menu
entry/tag_descriptions table, since TOML absorbs bare keys into the last
open table until a new header appears. show_rss_link was the only visibly
broken one (RSS link missing from homepage); the rest were masked by
matching template defaults.

Also replaced Tera's default() filter with explicit if/else where values
feed into JSON-LD, since default()'s fallback gets escaped in a way `safe`
can't undo. Added author/dateModified to BlogPosting list entries.

templates/posts.html

@@ -30,9 +30,14 @@ og_description %}{{ config.extra.posts_page_subtitle | default(value="Exploring
         "@type": "BlogPosting",
         "@id": "{{ page.permalink | safe }}#article",
         "headline": {{ page.title | json_encode | safe }},
-        "image": "{{ config.base_url | safe }}{{ page.extra.image | default(value="/images/og-image.png") | safe }}",
-        "url": "{{ page.permalink | safe }}"
+        "image": "{{ config.base_url | safe }}{% if page.extra.image %}{{ page.extra.image | safe }}{% else %}/images/og-image.png{% endif %}",
+        "url": "{{ page.permalink | safe }}",
+        "author": {
+          "@type": "Person",
+          "@id": "{{ config.base_url | safe }}/#author"
+        }
         {% if page.date %},"datePublished": "{{ page.date | date(format="%+") }}"{% endif %}
+        {% if page.updated %},"dateModified": "{{ page.updated | date(format="%+") }}"{% elif page.date %},"dateModified": "{{ page.date | date(format="%+") }}"{% endif %}
       }{% if not loop.last %},{% endif %}
       {% endfor %}
     ]