Show SABnzbd's queue in the header on every page

Adds a live download figure next to the header clock, present on
every page (not just Activity/System). It's populated by the same
/api/sabnzbd poll those pages already use, extended to run
unconditionally and to fill in a new #head-dl span — never a
server-rendered fetch, so an unreachable SABnzbd can't add latency to
pages that don't otherwise touch it. Quiet ink at "0 IN QUEUE" when
idle, live cyan with a speed reading once something's moving.

templates/_shell.html

@@ -42,6 +42,11 @@ provenance.
     <span class="tt-head-num">P{{ here[1] }}</span>
     <span class="tt-head-name">{{ here[2] }}</span>
     <span class="tt-entry" id="page-entry" hidden aria-live="polite"></span>
+    {#- SABnzbd's queue, on every page — the same live /api/sabnzbd poll
+        Activity and System already use, not a server-rendered fetch, so
+        an unreachable SABnzbd never adds latency to a page that doesn't
+        otherwise need it. Starts blank and fills in within a second. -#}
+    <span class="tt-head-dl" id="head-dl" aria-live="polite"></span>
     <span class="tt-head-clock" id="head-clock"></span>
   </header>