Refactor text utility classes to Tailwind CSS (#36703)

Replace Fomantic/custom CSS text utility classes with their Tailwind
equivalents:

- `.text.<color>` compound classes → `tw-text-<color>` classes
- `.text.small` (`font-size: 0.75em`) → `tw-text-xs` (11px)
- `.text.truncate` (`overflow-x: hidden; text-overflow: ellipsis;
white-space: nowrap; display: inline-block`) → `tw-inline-block
tw-truncate`

Remove the now-unused CSS rules from `base.css` and `dashboard.css`.

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

templates/repo/issue/milestone_issues.tmpl

@@ -36,7 +36,7 @@
 					{{else}}
 
 						{{if .Milestone.DeadlineString}}
-							<span{{if .IsOverdue}} class="text red"{{end}}>
+							<span{{if .IsOverdue}} class="tw-text-red"{{end}}>
 								{{svg "octicon-calendar"}}
 								{{DateUtils.AbsoluteShort (.Milestone.DeadlineString|DateUtils.ParseLegacy)}}
 							</span>