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.

static/css/teletext.css

@@ -167,6 +167,10 @@ a.tt-row:focus-visible .tt-row-sub { color: var(--tt-black); }
   text-transform: uppercase;
   min-width: 11ch;
 }
+/* Marks a row that leaves the app (System's Transmission links to each
+   service's own UI) — everywhere else a .tt-row stays inside Media
+   Centre, so leaving needs its own quiet signal. */
+.tt-row-ext { color: var(--ink-dim); }
 .tt-row-body { flex: 1; min-width: 0; }
 .tt-row-title {
   overflow-wrap: anywhere;
@@ -494,7 +498,7 @@ a.tt-row:focus-visible .tt-row-sub { color: var(--tt-black); }
 .fastext-top {
   margin: var(--row) auto calc(var(--row) * 1.5);
 }
-.fastext a {
+.fastext a, .fastext-here {
   display: flex;
   align-items: center;
   min-width: 0;
@@ -510,18 +514,25 @@ a.tt-row:focus-visible .tt-row-sub { color: var(--tt-black); }
 }
 /* The key's colour is carried by a solid cell at its head — the
    physical coloured button on the remote. */
-.fastext a > span {
+.fastext a > span, .fastext-here > span {
   min-width: 0;
   overflow: hidden;
   text-overflow: ellipsis;
 }
-.fastext a::before {
+.fastext a::before, .fastext-here::before {
   content: "";
   flex: 0 0 auto;
   width: 1.2ch;
   height: 1em;
   background: currentColor;
 }
+/* The page you're already on: present for orientation, same shape as
+   its neighbours, but not a link — clicking your own destination did
+   nothing useful, so now it isn't offered as a click at all. */
+.fastext-here {
+  color: var(--ink-dim);
+  cursor: default;
+}
 .fastext .k-red    { color: var(--alert); }
 .fastext .k-green  { color: var(--ok); }
 .fastext .k-yellow { color: var(--voice); }