feat: Refactor ScheduleModal to support interval scheduling with date input and deadline toggle

- Updated ChoreSchedule model to include anchor_date and interval_has_deadline.
- Refactored interval scheduling logic in scheduleUtils to use anchor_date.
- Introduced DateInputField component for selecting anchor dates in ScheduleModal.
- Enhanced ScheduleModal to include a stepper for interval days and a toggle for deadline.
- Updated tests for ScheduleModal and scheduleUtils to reflect new interval scheduling logic.
- Added DateInputField tests to ensure proper functionality and prop handling.

frontend/vue-app/src/components/shared/DateInputField.vue

@@ -0,0 +1,39 @@
+<template>
+  <input class="date-input-field" type="date" :value="modelValue" :min="min" @change="onChanged" />
+</template>
+
+<script setup lang="ts">
+const props = defineProps<{
+  modelValue: string
+  min?: string
+}>()
+
+const emit = defineEmits<{
+  (e: 'update:modelValue', value: string): void
+}>()
+
+function onChanged(event: Event) {
+  emit('update:modelValue', (event.target as HTMLInputElement).value)
+}
+</script>
+
+<style scoped>
+.date-input-field {
+  padding: 0.4rem 0.6rem;
+  border: 1.5px solid var(--kebab-menu-border, #bcc1c9);
+  border-radius: 6px;
+  background: var(--modal-bg, #fff);
+  color: var(--secondary, #7257b3);
+  font-size: 0.95rem;
+  font-weight: 600;
+  font-family: inherit;
+  cursor: pointer;
+  outline: none;
+  transition: border-color 0.15s;
+}
+
+.date-input-field:hover,
+.date-input-field:focus {
+  border-color: var(--btn-primary, #667eea);
+}
+</style>