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/tsconfig.json

@@ -5,7 +5,8 @@
     "moduleResolution": "bundler",
     "strict": true,
     "jsx": "preserve",
-    "lib": ["ES2020", "DOM"],
+    "lib": ["ES2020", "DOM", "DOM.Iterable"],
+    "types": ["vite/client"],
     "skipLibCheck": true,
     "paths": { "@/*": ["./src/*"] }
   },