feat(i18n): 21-language pipeline, landing/docs/API wiring, landing+API translations

Shared Claude Code translation pipeline (scripts/i18n, no API key) plus Astro/VitePress/Scalar i18n wiring. Landing and API reference translated into all 20 languages; docs i18n wiring + English source anchors. The translated docs markdown (apps/docs/<locale>/**, 3,620 files) follows in a companion PR because it exceeds GitHub's per-PR CI file limit.

apps/landing/astro.config.mjs

@@ -1,16 +1,54 @@
 import sitemap from "@astrojs/sitemap";
+import { SUPPORTED_LOCALES } from "@snapotter/shared";
 import tailwindcss from "@tailwindcss/vite";
 import { defineConfig } from "astro/config";
 
+const LOCALES = SUPPORTED_LOCALES.map((l) => l.code);
+
+// Tool DETAIL pages (/tools/<section>/<tool>/) are English-only, so they have no
+// localized variants. Match exactly two path segments after /tools/ (the /tools/
+// index and /tools/<section>/ section pages have zero and one, and stay localized).
+const TOOL_DETAIL_PATH = /^\/tools\/(?:image|video|audio|pdf|files)\/[^/]+\/$/;
+
+function isToolDetailUrl(url) {
+  try {
+    return TOOL_DETAIL_PATH.test(new URL(url).pathname);
+  } catch {
+    return false;
+  }
+}
+
 export default defineConfig({
   site: "https://snapotter.com",
   output: "static",
+  i18n: {
+    defaultLocale: "en",
+    locales: LOCALES,
+    routing: {
+      prefixDefaultLocale: false,
+    },
+  },
   devToolbar: {
     enabled: !process.env.PLAYWRIGHT,
   },
   integrations: [
     sitemap({
       filter: (page) => !page.includes("/404"),
+      i18n: {
+        defaultLocale: "en",
+        locales: Object.fromEntries(LOCALES.map((c) => [c, c])),
+      },
+      // English-only tool-detail pages have no localized variants, so drop the
+      // hreflang alternates the i18n integration would otherwise emit for them
+      // (they would point at non-existent /de/tools/<tool>/ URLs). All other
+      // pages keep their alternates.
+      serialize(item) {
+        if (isToolDetailUrl(item.url)) {
+          const { links, ...rest } = item;
+          return rest;
+        }
+        return item;
+      },
     }),
   ],
   vite: {