dashboard: redesign on Bootstrap 5, split page into external css/js

Page now uses Bootstrap 5.3 color modes (data-bs-theme), Bootstrap Icons,
and Bootstrap modals. Custom code shrinks to dashboard.css (app tokens,
score bar, signal accents) and dashboard.js (rendering/polling logic,
ported verbatim from the inline script). dashboard_publisher copies the
new css/js into the docroot; all 381 tests still green.

test_all.py

@@ -5670,7 +5670,7 @@ def _():
         pub = dpub.DashboardPublisher(out_dir=tmp)
         pub.install_static()
         assert sorted(os.listdir(tmp)) == \
-            ["dashboard.html", "news_summary.php"]  # both installed, no .tmp
+            ["dashboard.css", "dashboard.html", "dashboard.js", "news_summary.php"]  # all installed, no .tmp
 
 
 @test("dashboard: publish is atomic and leaves no temp file")