feat(07-01): create useWeightUnit convenience hook

- Wraps useSetting("weightUnit") with typed WeightUnit return
- Validates stored value against known units (g, oz, lb, kg)
- Defaults to "g" when no setting exists (backward compatible)
- Fix config.json formatting (tabs per biome config)

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

.planning/config.json

@@ -5,7 +5,7 @@
 	"commit_docs": true,
 	"model_profile": "quality",
 	"workflow": {
-		"research": false,
+		"research": true,
 		"plan_check": true,
 		"verifier": true,
 		"nyquist_validation": true,