fix(ui): add dark mode styles to amber warning boxes app-wide

Amber alert boxes were missing dark: variants, rendering as muddy
dark-orange in dark mode with near-unreadable text. Fixed in:
- VacationClient (Pending Approvals banner)
- VacationModal (conflict warning)
- ResourceDetail (load error)
- SkillMatrixUpload (replace warning)
- AllocationModal (open demand toggle)
- ProjectWizard (budget bar, post-creation warnings)

Pattern: bg-amber-50 → dark:bg-amber-950/30, border-amber-200 →
dark:border-amber-800, text-amber-7/800 → dark:text-amber-300/400

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/src/components/resources/ResourceDetail.tsx

@@ -167,7 +167,7 @@ export function ResourceDetail({ resourceId }: ResourceDetailProps) {
   if (error || !resource) {
     return (
       <div className="p-6">
-        <div className="bg-amber-50 border border-amber-200 rounded-xl p-6 text-amber-800 text-sm">
+        <div className="bg-amber-50 dark:bg-amber-950/30 border border-amber-200 dark:border-amber-800 rounded-xl p-6 text-amber-800 dark:text-amber-300 text-sm">
           This resource could not be loaded right now.{" "}
           <Link href="/resources" className="underline">Back to resources</Link>
         </div>