feat: add collapsible subtasks in list view

- Add chevron toggle button for each task with subtasks
- Add global collapse/expand all button in filter toolbar
- Use provide/inject for global collapse state
- Smooth rotation animation on collapse toggle

Closes #126

frontend/src/components/project/views/ProjectList.vue

@@ -17,6 +17,14 @@
 					:project-id="projectId"
 					@update:modelValue="loadTasks()"
 				/>
+				<button
+					class="collapse-all-btn"
+					:class="{ 'is-collapsed': collapseAll }"
+					@click="toggleCollapseAll"
+					:title="collapseAll ? 'Expand all subtasks' : 'Collapse all subtasks'"
+				>
+					<Icon :icon="collapseAll ? 'chevron-right' : 'chevron-down'" />
+				</button>
 			</div>
 		</template>
 
@@ -101,7 +109,7 @@
 
 
 <script setup lang="ts">
-import {ref, computed, nextTick, onMounted, onBeforeUnmount, watch, toRef} from 'vue'
+import {ref, computed, nextTick, onMounted, onBeforeUnmount, watch, toRef, provide} from 'vue'
 import draggable from 'zhyswan-vuedraggable'
 
 import ProjectWrapper from '@/components/project/ProjectWrapper.vue'
@@ -143,6 +151,14 @@ const ctaVisible = ref(false)
 
 const drag = ref(false)
 
+// Collapse all subtasks
+const collapseAll = ref(false)
+provide('collapseAll', collapseAll)
+
+function toggleCollapseAll() {
+	collapseAll.value = !collapseAll.value
+}
+
 const {
 	tasks: allTasks,
 	loading,
@@ -368,6 +384,31 @@ onBeforeUnmount(() => {
 	}
 }
 
+.collapse-all-btn {
+	display: flex;
+	align-items: center;
+	justify-content: center;
+	inline-size: 32px;
+	block-size: 32px;
+	border: 1px solid var(--grey-300);
+	background: var(--white);
+	color: var(--grey-500);
+	cursor: pointer;
+	border-radius: $radius;
+	transition: all 0.2s ease;
+
+	&:hover {
+		color: var(--grey-700);
+		background: var(--grey-100);
+		border-color: var(--grey-400);
+	}
+
+	&.is-collapsed {
+		color: var(--primary);
+		border-color: var(--primary);
+	}
+}
+
 .tasks {
 	padding: .5rem;
 }

frontend/src/components/tasks/partials/SingleTaskInProject.vue

@@ -178,8 +178,17 @@
 				/>
 			</BaseButton>
 			<slot />
+			<button
+				v-if="hasSubtasks"
+				class="collapse-toggle"
+				:class="{ 'is-collapsed': isCollapsed }"
+				@click.stop="toggleCollapse"
+				:aria-label="isCollapsed ? 'Expand subtasks' : 'Collapse subtasks'"
+			>
+				<Icon icon="chevron-down" />
+			</button>
 		</div>
-		<template v-if="typeof task.relatedTasks?.subtask !== 'undefined'">
+		<template v-if="hasSubtasks && !isCollapsed">
 			<template v-for="subtask in task.relatedTasks.subtask">
 				<template v-if="getTaskById(subtask.id)">
 					<single-task-in-project
@@ -197,7 +206,7 @@
 </template>
 
 <script setup lang="ts">
-import {ref, watch, shallowReactive, onMounted, computed} from 'vue'
+import {ref, watch, shallowReactive, onMounted, computed, inject, type Ref} from 'vue'
 import {useI18n} from 'vue-i18n'
 
 import TaskModel, {getHexColor} from '@/models/task'
@@ -246,6 +255,24 @@ const props = withDefaults(defineProps<{
 	allTasks: () => [],
 })
 
+// Collapse state
+const isCollapsed = ref(false)
+const collapseAll = inject<Ref<boolean>>('collapseAll', ref(false))
+
+// Watch for global collapse/expand all
+watch(collapseAll, (newVal) => {
+	isCollapsed.value = newVal
+})
+
+const hasSubtasks = computed(() => {
+	return typeof props.theTask.relatedTasks?.subtask !== 'undefined' && 
+		props.theTask.relatedTasks.subtask.length > 0
+})
+
+function toggleCollapse() {
+	isCollapsed.value = !isCollapsed.value
+}
+
 const emit = defineEmits<{
 	'taskUpdated': [task: ITask],
 }>()
@@ -597,6 +624,30 @@ defineExpose({
 	margin-inline-start: 1.75rem;
 }
 
+.collapse-toggle {
+	display: flex;
+	align-items: center;
+	justify-content: center;
+	inline-size: 20px;
+	block-size: 20px;
+	border: none;
+	background: transparent;
+	color: var(--grey-400);
+	cursor: pointer;
+	border-radius: $radius;
+	transition: transform 0.2s ease, color 0.2s ease;
+	margin-inline-start: 0.5rem;
+
+	&:hover {
+		color: var(--grey-600);
+		background: var(--grey-100);
+	}
+
+	&.is-collapsed {
+		transform: rotate(-90deg);
+	}
+}
+
 :deep(.popup) {
 	border-radius: $radius;
 	background-color: var(--white);