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/allocations/AllocationModal.tsx

@@ -291,7 +291,7 @@ export function AllocationModal({ allocation, onClose, onSuccess }: AllocationMo
         {/* Form */}
         <form onSubmit={handleSubmit} className="px-6 py-5 space-y-4">
           {/* Demand toggle */}
-          <div className="flex items-center gap-3 p-3 bg-amber-50 border border-amber-200 rounded-lg">
+          <div className="flex items-center gap-3 p-3 bg-amber-50 dark:bg-amber-950/30 border border-amber-200 dark:border-amber-800 rounded-lg">
             <label className="flex items-center gap-2 text-sm cursor-pointer select-none flex-1">
               <input
                 type="checkbox"