Fix design-review findings: self-linking nav, colour reuse, badge clash

From a frontend-design skill review of all pages, after last session's
chrome changes (top nav, all-tabs nav, header download row):

- The top/closing fastext nav now mutes the entry matching the current
  page (.fastext-here) instead of rendering a link to the page you're
  already on.
- Colour in fastext is reserved for the four entries that still carry
  a real keyboard shortcut (r/g/y/c); the other four render in plain
  ink instead of reusing k-red/k-green/k-yellow/k-cyan for a
  destination they don't semantically mean (e.g. Requests no longer
  shows in the app's own off-air red).
- Discover's badge slot meant "critic score" on its RT tiles and
  "release year" on its new search tiles at the same time. Search
  results (Library and Discover) now fold year/held-state into the
  sub-line instead, leaving the badge exclusively for critic score.
- System's Transmission rows now carry a small ↗ glyph, since they
  silently changed from in-app navigation to opening each service's
  own UI in a new tab with no visual cue.
- SABnzbd polling drops from 5s to 20s on pages that only show the
  header's download figure (everywhere but System/Activity, which
  keep 5s for their own live figures) — a number idle 95% of the time
  doesn't need second-level freshness sitewide.
- DESIGN.md's fastext description, now-current per this session's nav
  changes.

templates/_shell.html

@@ -58,9 +58,13 @@ provenance.
   {% set fastext_nav %}
   <nav class="fastext{{ '' if page == 'home' else ' fastext-top' }}" aria-label="Fastext">
     {% for cls, key, num, label, href in fastext %}
+    {% if num == here[1] %}
+    <span class="fastext-here" aria-current="page"><span>{{ num }} {{ label }}</span></span>
+    {% else %}
     <a class="{{ cls }}" href="{{ request.script_root }}{{ href }}" data-key="{{ key }}">
       <span>{{ num }} {{ label }}</span>
     </a>
+    {% endif %}
     {% endfor %}
   </nav>
   {% endset %}