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/src/lib/i18n-page.ts

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+// apps/landing/src/lib/i18n-page.ts
+import { getRelativeLocaleUrl } from "astro:i18n";
+import { isRtl, LANDING_LOCALES } from "@/i18n";
+
+/** Absolute site origin used for canonical + hreflang hrefs. */
+export const SITE = "https://snapotter.com";
+
+/** Prefix a path for a locale ("/faq" -> "/de/faq" for de, "/faq" for en). */
+export function localizeHref(locale: string, path: string): string {
+  const clean = path.startsWith("/") ? path.slice(1) : path;
+  return getRelativeLocaleUrl(locale, clean);
+}
+
+/** Direction attribute for the current locale. */
+export function dirFor(locale: string): "ltr" | "rtl" {
+  return isRtl(locale) ? "rtl" : "ltr";
+}
+
+/**
+ * Reciprocal hreflang alternates for one page path (same path across all locales),
+ * plus an x-default pointing at English.
+ */
+export function altLinks(path: string): Array<{ hreflang: string; href: string }> {
+  const links = LANDING_LOCALES.map((code) => ({
+    hreflang: code,
+    href: `${SITE}${localizeHref(code, path)}`,
+  }));
+  links.push({ hreflang: "x-default", href: `${SITE}${localizeHref("en", path)}` });
+  return links;
+}
+
+/** BCP-47-ish og:locale value ("de" -> "de", "pt-BR" -> "pt_BR"). */
+export function ogLocale(locale: string): string {
+  return locale.replace("-", "_");
+}

apps/landing/src/lib/seo-content.ts

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+// apps/landing/src/lib/seo-content.ts
+import { SUPPORTED_LOCALES } from "@snapotter/shared";
+
+// Eager glob of every generated locale alternatives file. Missing files simply
+// do not appear, so lookups fall back to the English source passed by the caller.
+// Only the alternatives pages are localized; tool-detail pages are English-only.
+const altFiles = import.meta.glob<{ default: Record<string, { text: string }> }>(
+  "../data/i18n/alternatives.*.json",
+  { eager: true },
+);
+
+function catalog(
+  files: Record<string, { default: Record<string, { text: string }> }>,
+  base: string,
+  locale: string,
+) {
+  return files[`../data/i18n/${base}.${locale}.json`]?.default ?? {};
+}
+
+/** Native/tool locale codes, from shared. */
+export const SEO_LOCALES = SUPPORTED_LOCALES.map((l) => l.code);
+
+/**
+ * Look up a translated alternatives string by id, falling back to the English source.
+ * `id` matches the landing-seo adapter ids ("alt:<slug>:<field>").
+ */
+export function seoText(locale: string, id: string, englishFallback: string): string {
+  if (locale === "en") return englishFallback;
+  return catalog(altFiles, "alternatives", locale)[id]?.text ?? englishFallback;
+}
+
+/** Translate an indexed array field, falling back per-item to the English source array. */
+export function seoArray(locale: string, base: string, field: string, english: string[]): string[] {
+  if (locale === "en") return english;
+  return english.map((en, i) => seoText(locale, `${base}:${field}.${i}`, en));
+}

apps/landing/src/lib/tool-strings.ts

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+// apps/landing/src/lib/tool-strings.ts
+import { loadTranslations } from "@snapotter/shared";
+
+/** Translated tool name/description for a locale, keyed by toolId. */
+export async function loadToolStrings(
+  locale: string,
+): Promise<Record<string, { name: string; description: string }>> {
+  const dict = await loadTranslations(locale);
+  return (
+    (dict as unknown as { tools?: Record<string, { name: string; description: string }> }).tools ??
+    {}
+  );
+}