feat: add search input field with placeholder text to main header component

Introduce a new sidebar search template that renders a filter form with a hidden input loop and a text input bound to the `_placeholder` variable, supporting GET requests and a maxlength of 100 characters.

devplacepy/templates/_sidebar_search.html

@@ -0,0 +1,7 @@
+<div class="sidebar-heading">Filter</div>
+<div class="sidebar-section">
+    <form method="GET" action="{{ _action }}">
+        {% for _key, _value in (_hidden or {}).items() %}{% if _value %}<input type="hidden" name="{{ _key }}" value="{{ _value }}">{% endif %}{% endfor %}
+        <input type="text" name="search" placeholder="{{ _placeholder }}" value="{{ search or '' }}" aria-label="{{ _placeholder }}" maxlength="100">
+    </form>
+</div>