fix: Alpine.js JSON parsing error in x-data HTML attributes

Add tojson_attr Jinja2 filter that escapes double quotes to "
for safe embedding in HTML attributes. The browser decodes entities
before Alpine.js evaluates, so JSON parses correctly.

Replaces |tojson with |tojson_attr in x-data attributes (select_recipe,
recipe_list, base flash messages). Script tag usages are unaffected.

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

client/templates/base.html

@@ -90,7 +90,7 @@
     {% with messages = get_flashed_messages(with_categories=true) %}
       {% if messages %}
         <div class="fixed top-16 right-4 z-50 flex flex-col gap-2 max-w-md w-full"
-             x-data="{ messages: {{ messages|tojson }} }"
+             x-data="{ messages: {{ messages|tojson_attr }} }"
              x-init="setTimeout(() => { $el.remove() }, 8000)">
           {% for category, message in messages %}
             <div x-data="{ show: true }"