feat(frontend): illustrated null states and author rewards fallback

Add aksrt_get_null / widget null helpers with null.svg, use them in
sidebar widgets, comments empty, and related posts; when PM is off show
post rewards entry on the single author bar.

assets/img/null.svg

@@ -0,0 +1,9 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="280" height="200" viewBox="0 0 280 200" fill="none" aria-hidden="true">
+  <rect width="280" height="200" fill="transparent"/>
+  <ellipse cx="140" cy="168" rx="72" ry="10" fill="#E8E8E8" opacity=".55"/>
+  <rect x="78" y="58" width="124" height="96" rx="14" fill="#F3F4F6" stroke="#D1D5DB" stroke-width="2"/>
+  <path d="M98 92h84M98 110h62M98 128h48" stroke="#C4C9D1" stroke-width="6" stroke-linecap="round"/>
+  <circle cx="188" cy="70" r="22" fill="#FFF7ED" stroke="#FDBA74" stroke-width="2"/>
+  <path d="M188 60v12M182 72h12" stroke="#F59E0B" stroke-width="3" stroke-linecap="round"/>
+  <path d="M104 48c8-14 28-18 40-8" stroke="#9CA3AF" stroke-width="3" stroke-linecap="round" fill="none"/>
+</svg>