feat(catalog): wire Fiat (p5fiat) browse — families→models two-step + de routing

Fiat catalog browse returned 0 models because the generic single-endpoint P5
model-list flow can't express Fiat's two-level hierarchy and the drill defaulted
to the tr account (Fiat is licensed only on de-708171).

- fetchVehicleList: dispatch fiatp_parts/fiatt_parts to new fetchFiatVehicleList,
  which expands modelOverview (34 families) → models?modelFamily=N (model codes +
  year ranges) into flat catalog vehicles whose catalogPath is the maingroups
  endpoint. Verified live: 34 families → 123 models, end-to-end drill to parts+image.
- fetchMainGroups / fetchP5Restrictions: account-aware (resolveAccount → de + DE
  proxy for Fiat; tr unchanged for every other P5 brand) so browse maingroups no
  longer hit the tr demo/empty page.
- web: case-insensitive "/maingroup" gate so Fiat (lowercase /mdl/maingroups,
  already a maingroups endpoint) skips the empty restriction selector and loads
  categories directly.
- formatFiatYear: "(2016,2020)" → "2016-2020"; +unit tests.

Subgroups/parts/images already resolve account→de for Fiat (unchanged).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/web/src/routes/dashboard/catalog_/$brandName_/$modelId/index.tsx

@@ -72,7 +72,9 @@ function CatalogVehiclePage() {
   const isP5WithRestrictions =
     vehicle?.architecture === "P5_MODERN" &&
     !!vehicle?.catalogPath &&
-    !vehicle.catalogPath.includes("/mainGroup");
+    // Case-insensitive: Fiat's catalogPath is already a maingroups endpoint
+    // (/extern/groups/mdl/maingroups) → no restriction step, categories load directly.
+    !/\/maingroup/i.test(vehicle.catalogPath);
   const showPsaVariantSelector = isPsa && !hasVariant;
   const showFordVariantSelector = isP4Legacy && !hasVariant;
   const showP5RestrictionSelector = isP5WithRestrictions && !hasVariant;