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/package.json

@@ -4,8 +4,8 @@
   "description": "Standalone offline HTML/JS tool for building APDU commands for SIM, USIM, and GlobalPlatform RAM, plus encoding conversions.",
   "main": "index.js",
   "scripts": {
-    "build": "npx tailwindcss -i src/style.css -o style.css --config tailwind.config.js",
-    "build:prod": "NODE_ENV=production npx tailwindcss -i src/style.css -o style.css --config tailwind.config.js --minify",
+    "build": "npx tailwindcss -i src/style.css -o style.css --config tailwind.config.js && cat src/contrast.css >> style.css",
+    "build:prod": "NODE_ENV=production npx tailwindcss -i src/style.css -o style.css --config tailwind.config.js --minify && cat src/contrast.css >> style.css",
     "test": "node --test"
   },
   "repository": {