feat: add enhanced SABnzbd download dashboard with remaining data calculation and visual redesign

Add parse_size_bytes() and format_bytes() helper functions to convert and format file sizes. Calculate total remaining bytes from queue and include active_count, history_count, remaining_bytes, and formatted remaining in downloads API response. Update error responses to include new fields.

Add media-mark.svg favicon with gradient background, golden lines, and green checkmark. Replace text logo with inline

static/media-mark.svg

@@ -0,0 +1,12 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64">
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0" stop-color="#293542"/>
+      <stop offset="1" stop-color="#15191f"/>
+    </linearGradient>
+    <filter id="glow"><feGaussianBlur stdDeviation="2.2" result="b"/><feMerge><feMergeNode in="b"/><feMergeNode in="SourceGraphic"/></feMerge></filter>
+  </defs>
+  <rect x="5" y="5" width="54" height="54" rx="15" fill="url(#bg)" stroke="#60a9e6" stroke-width="2.5"/>
+  <path d="M17 21h30M17 31h19M17 41h30" fill="none" stroke="#f4c430" stroke-width="4" stroke-linecap="round"/>
+  <circle cx="46" cy="31" r="7" fill="#159447" stroke="#c9ffe0" stroke-width="2" filter="url(#glow)"/>
+</svg>