fix(frontend): add HTMX filter submission for sidebar radio buttons

Wrap sidebar time/type filters in a form with HTMX attributes so
filter changes trigger partial page updates instead of full reload.

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

internal/views/templates/results.html

@@ -20,7 +20,10 @@
       {{end}}
     </nav>
 
-    <div class="sidebar-filters">
+    <form method="GET" action="/search" class="sidebar-filters"
+      hx-get="/search" hx-target="#results" hx-trigger="change" hx-swap="innerHTML">
+      <input type="hidden" name="q" value="{{.Query | urlquery}}">
+      <input type="hidden" name="category" value="{{.ActiveCategory}}">
       <div class="sidebar-filter-group">
         <div class="sidebar-filter-label">Time</div>
         {{range .TimeFilters}}
@@ -39,7 +42,7 @@
         </label>
         {{end}}
       </div>
-    </div>
+    </form>
   </aside>
 
   <!-- Center Column -->