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/api/tasks.ts

@@ -0,0 +1,20 @@
+import client from './client'
+
+export interface Task {
+  id?: number
+  titre: string
+  description?: string
+  garden_id?: number
+  priorite: string
+  echeance?: string
+  statut: string
+}
+
+export const tasksApi = {
+  list: (params?: { statut?: string; garden_id?: number }) =>
+    client.get<Task[]>('/api/tasks', { params }).then(r => r.data),
+  get: (id: number) => client.get<Task>(`/api/tasks/${id}`).then(r => r.data),
+  create: (t: Partial<Task>) => client.post<Task>('/api/tasks', t).then(r => r.data),
+  update: (id: number, t: Partial<Task>) => client.put<Task>(`/api/tasks/${id}`, t).then(r => r.data),
+  delete: (id: number) => client.delete(`/api/tasks/${id}`),
+}