Add widget registry for universal UI decoupling

Introduces a widget system where domains register UI fragments into
named slots (container_nav, container_card, account_page, account_link).
Host apps iterate widgets generically without naming any domain.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

browser/templates/_widgets/container_card/calendar_entries.html

@@ -0,0 +1,36 @@
+{# Associated entries on blog listing cards — loaded via widget registry #}
+{% set widget_entries = post[w.context_key] if post[w.context_key] is defined else [] %}
+{% if widget_entries %}
+  <div class="mt-4 mb-2">
+    <h3 class="text-sm font-semibold text-stone-700 mb-2 px-2">Events:</h3>
+    <div class="overflow-x-auto scrollbar-hide" style="scroll-behavior: smooth;">
+      <div class="flex gap-2 px-2">
+        {% for entry in widget_entries %}
+          {% set _entry_path = '/' + post.slug + '/calendars/' + entry.calendar_slug + '/' + entry.start_at.year|string + '/' + entry.start_at.month|string + '/' + entry.start_at.day|string + '/entries/' + entry.id|string + '/' %}
+          <a
+            href="{{ events_url(_entry_path) }}"
+            class="flex flex-col gap-1 px-3 py-2 bg-stone-50 hover:bg-stone-100 rounded border border-stone-200 transition text-sm whitespace-nowrap flex-shrink-0 min-w-[180px]">
+            <div class="font-medium text-stone-900 truncate">{{ entry.name }}</div>
+            <div class="text-xs text-stone-600">
+              {{ entry.start_at.strftime('%a, %b %d') }}
+            </div>
+            <div class="text-xs text-stone-500">
+              {{ entry.start_at.strftime('%H:%M') }}
+              {% if entry.end_at %} – {{ entry.end_at.strftime('%H:%M') }}{% endif %}
+            </div>
+          </a>
+        {% endfor %}
+      </div>
+    </div>
+  </div>
+
+  <style>
+  .scrollbar-hide::-webkit-scrollbar {
+    display: none;
+  }
+  .scrollbar-hide {
+    -ms-overflow-style: none;
+    scrollbar-width: none;
+  }
+  </style>
+{% endif %}

browser/templates/_widgets/container_nav/calendar_entries.html

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+{# Calendar entries nav items — loaded via widget registry #}
+{% set entry_list = ctx.entries if ctx.entries is defined else [] %}
+{% set current_page = ctx.page if ctx.page is defined else 1 %}
+{% set has_more_entries = ctx.has_more if ctx.has_more is defined else False %}
+
+{% for entry in entry_list %}
+  {% set _entry_path = '/' + post.slug + '/calendars/' + entry.calendar_slug + '/' + entry.start_at.year|string + '/' + entry.start_at.month|string + '/' + entry.start_at.day|string + '/entries/' + entry.id|string + '/' %}
+  <a
+    href="{{ events_url(_entry_path) }}"
+    class="{{styles.nav_button_less_pad}}"
+  >
+    {% if post.feature_image %}
+      <img src="{{ post.feature_image }}"
+           alt="{{ post.title }}"
+           class="w-8 h-8 rounded object-cover flex-shrink-0" />
+    {% else %}
+      <div class="w-8 h-8 rounded bg-stone-200 flex-shrink-0"></div>
+    {% endif %}
+    <div class="flex-1 min-w-0">
+      <div class="font-medium truncate">{{ entry.name }}</div>
+      <div class="text-xs text-stone-600 truncate">
+        {{ entry.start_at.strftime('%b %d, %Y at %H:%M') }}
+        {% if entry.end_at %} – {{ entry.end_at.strftime('%H:%M') }}{% endif %}
+      </div>
+    </div>
+  </a>
+{% endfor %}
+
+{# Load more entries one at a time until container is full #}
+{% if has_more_entries %}
+  <div id="entries-load-sentinel-{{ current_page }}"
+       hx-get="{{ url_for('blog.post.widget_paginate', slug=post.slug, widget_domain='calendar', page=current_page + 1) }}"
+       hx-trigger="intersect once"
+       hx-swap="beforebegin"
+       _="on htmx:afterRequest trigger scroll on #associated-entries-container"
+       class="flex-shrink-0 w-1">
+  </div>
+{% endif %}

browser/templates/_widgets/container_nav/calendar_links.html

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+{# Calendar link nav items — loaded via widget registry #}
+{% for calendar in ctx.calendars %}
+  {% set local_href=events_url('/' + post.slug + '/calendars/' + calendar.slug + '/') %}
+  <a
+    href="{{ local_href }}"
+    class="{{styles.nav_button_less_pad}}">
+    <i class="fa fa-calendar" aria-hidden="true"></i>
+    <div>{{calendar.name}}</div>
+  </a>
+{% endfor %}

browser/templates/_widgets/container_nav/market_links.html

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+{# Market link nav items — loaded via widget registry #}
+{% for m in ctx.markets %}
+  <a
+    href="{{ market_url('/' + post.slug + '/' + m.slug + '/') }}"
+    class="{{styles.nav_button_less_pad}}">
+    <i class="fa fa-shopping-bag" aria-hidden="true"></i>
+    <div>{{m.name}}</div>
+  </a>
+{% endfor %}