added expense state (view, edit, add) logic

src/app/components/expense/expense.component.scss

@@ -0,0 +1,43 @@
+.expense-container {
+  width: 100%;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+
+  mat-card {
+    max-width: 800px;
+    width: 100%;
+  }
+}
+
+mat-card-header {
+  padding-bottom: 1rem;
+
+  .expense-header {
+    width: 100%;
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+  }
+}
+
+.expense-content {
+  display: grid;
+}
+
+mat-form-field {
+  width: 100%;
+}
+
+@media (min-width: 550px) {
+  .expense-content {
+    grid-template-columns: 1fr 1fr;
+    gap: 1rem;
+  }
+}
+
+@media (min-width: 800px) {
+  .expense-content {
+    grid-template-columns: 1fr 1fr 1fr;
+  }
+}