REFACTORED - Flatten portfolio category routes to /portfolio/[slug]

Merge the category and project routes under a single /portfolio/[slug]
segment via resolvePortfolioSlug (category wins over project on a slug
clash). Removes the /portfolio/category/... prefix from links, redirect,
and sitemap. Adds resolver integration tests.

tests/component/portfolio-category-filter.test.tsx

@@ -35,7 +35,7 @@ describe("PortfolioCategoryFilter", () => {
     expect(screen.getByRole("link", { name: "All" })).toHaveAttribute("href", "/en/portfolio");
     expect(screen.getByRole("link", { name: "Branding" })).toHaveAttribute(
       "href",
-      "/en/portfolio/category/branding",
+      "/en/portfolio/branding",
     );
   });
 
@@ -51,7 +51,7 @@ describe("PortfolioCategoryFilter", () => {
     // German locale on the default locale -> unprefixed paths and German labels
     expect(screen.getByRole("link", { name: "Marke" })).toHaveAttribute(
       "href",
-      "/portfolio/category/branding",
+      "/portfolio/branding",
     );
     expect(screen.getByRole("link", { name: "Alle" })).toHaveAttribute("href", "/portfolio");
   });