chore(design-system): replace dead Bootstrap classes with Sure tokens (#1621)

Sure uses Tailwind v4 with the design system tokens but several views
still carried Bootstrap-style class names that don't render anything
because no Bootstrap stylesheet is loaded. They're effectively dead
markup.

Replacements:
- text-muted, text-muted-foreground -> text-subdued
- bg-light -> bg-surface
- font-italic -> italic
- text-uppercase -> uppercase
- font-weight-bold -> font-bold

Touched files:
- app/views/doorkeeper/applications/_form.html.erb
- app/views/doorkeeper/applications/show.html.erb
- app/views/pages/privacy.html.erb
- app/views/pages/terms.html.erb
- app/views/pages/redis_configuration_error.html.erb
- app/views/settings/providers/_mercury_panel.html.erb

Also tightening application.css:
- The .hw-combobox__label rule used raw text-gray-500 / text-gray-400
  via @apply. Now uses the text-secondary / text-subdued tokens so the
  combobox label responds to the theme.
- Custom scrollbar thumbs in .windows and .scrollbar used hardcoded
  #d6d6d6 / #a6a6a6 hex values. Now reference var(--color-gray-300) /
  var(--color-gray-400). Slight color shift (the hex values were close
  to but not identical to those tokens), so this needs a quick visual
  check.

And reports/print.html.erb had four <span style="color: #666"> elements
on the metric cards. Replaced with class="text-secondary" merged into
the existing tufte-metric-card-change class, so print uses the same
secondary-text color the rest of the app uses.

app/assets/tailwind/application.css

@@ -56,7 +56,7 @@
   }
 
   .hw-combobox__label {
-    @apply block text-xs text-gray-500 peer-disabled:text-gray-400;
+    @apply block text-xs text-secondary peer-disabled:text-subdued;
   }
   
   .hw-combobox__option {
@@ -155,12 +155,12 @@
   }
 
   ::-webkit-scrollbar-thumb {
-    background: #d6d6d6;
+    background: var(--color-gray-300);
     border-radius: 10px;
   }
 
   ::-webkit-scrollbar-thumb:hover {
-    background: #a6a6a6;
+    background: var(--color-gray-400);
   }  
 }
 
@@ -170,12 +170,12 @@
   }
 
   &::-webkit-scrollbar-thumb {
-    background: #d6d6d6;
+    background: var(--color-gray-300);
     border-radius: 10px;
   }
 
   &::-webkit-scrollbar-thumb:hover {
-    background: #a6a6a6;
+    background: var(--color-gray-400);
   }
 }