feat: replace native confirm() dialogs with custom modal component

- Add ModalProvider context and useConfirm hook (confirm-modal.tsx)
- Support danger/warning/info variants with icons and colors
- Animated backdrop and dialog with CSS keyframes
- Accessible: aria-modal, role=dialog, ESC to close, auto-focus
- Replace all 6 native confirm() calls across dashboard pages
- Integrate ModalProvider in app providers

frontend/src/app/globals.css

@@ -138,3 +138,19 @@
 .animate-slide-up {
   animation: slideUp 0.4s ease-out;
 }
+
+/* ─── Modal animations ───────────────────────────────── */
+@keyframes modalBackdropIn {
+  from { opacity: 0; }
+  to { opacity: 1; }
+}
+@keyframes modalEnter {
+  from { opacity: 0; transform: scale(0.95) translateY(8px); }
+  to { opacity: 1; transform: scale(1) translateY(0); }
+}
+.animate-modal-backdrop {
+  animation: modalBackdropIn 0.15s ease-out;
+}
+.animate-modal-enter {
+  animation: modalEnter 0.2s ease-out;
+}