feat(frontend): API layer + stores Pinia

Ajoute le client Axios, les modules API (gardens, varieties, plantings,
tasks) et les stores Pinia correspondants. Corrige tsconfig.json pour
inclure vite/client et DOM.Iterable.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/stores/tasks.ts

@@ -0,0 +1,34 @@
+import { defineStore } from 'pinia'
+import { ref } from 'vue'
+import { tasksApi, type Task } from '@/api/tasks'
+
+export const useTasksStore = defineStore('tasks', () => {
+  const tasks = ref<Task[]>([])
+  const loading = ref(false)
+
+  async function fetchAll(params?: { statut?: string; garden_id?: number }) {
+    loading.value = true
+    tasks.value = await tasksApi.list(params)
+    loading.value = false
+  }
+
+  async function create(t: Partial<Task>) {
+    const created = await tasksApi.create(t)
+    tasks.value.push(created)
+    return created
+  }
+
+  async function updateStatut(id: number, statut: string) {
+    const t = tasks.value.find(t => t.id === id)
+    if (!t) return
+    const updated = await tasksApi.update(id, { ...t, statut })
+    Object.assign(t, updated)
+  }
+
+  async function remove(id: number) {
+    await tasksApi.delete(id)
+    tasks.value = tasks.value.filter(t => t.id !== id)
+  }
+
+  return { tasks, loading, fetchAll, create, updateStatut, remove }
+})