theme: brighten gray text and borders in dark mode

Elements using bare text-gray-300/400/500/600 (proactive log ids and
timestamps, expand markers, event codes, ...) kept the light-theme gray
values in dark mode, which made them hard to read on slate-800/900.

- dark .text-gray-*:not([class*='dark:text-']) rules map them to the same
  level as the other dark muted text (slate-300 #cbd5e1, slate-200/100 for
  gray-700/800)
- dark:text-gray-400/600 and dark:text-slate-500 normalized to #cbd5e1
- dark:border-slate-600/700/800 (+ /50) brightened one step
- light theme unchanged
- the whole contrast block moved to src/contrast.css, appended by the
  npm build scripts after Tailwind so it stays reproducible; SW cache
  v95 -> v96

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'otaman-v95';
+const CACHE = 'otaman-v96';
 const URLS = [
   'index.html',
   'help.html',