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.

tests/e2e-docs/fixtures/seed-de-locale.mjs

@@ -0,0 +1,40 @@
+// tests/e2e-docs/fixtures/seed-de-locale.mjs
+import { rm } from "node:fs/promises";
+import { join } from "node:path";
+import { fileURLToPath } from "node:url";
+import { createDocsAdapter } from "../../../scripts/i18n/adapters/docs-md.mjs";
+
+const DOCS = fileURLToPath(new URL("../../../apps/docs", import.meta.url));
+// The e2e asserts against these; only they get the [DE] heading prefix. Every
+// other file is still written under /de/ (English body) so the seeded German
+// tree has no dead internal links and VitePress builds it cleanly.
+const PICK = new Set(["index.md", "guide/getting-started.md", "guide/configuration.md"]);
+
+async function main() {
+  await rm(join(DOCS, "de"), { recursive: true, force: true });
+  const adapter = createDocsAdapter({ root: DOCS });
+  const units = await adapter.extract();
+  const entries = new Map();
+  for (const u of units) {
+    // Deterministic pseudo-translation: PICK pages get a [DE] ATX-heading prefix;
+    // the rest keep their English body. Masked tokens, anchors, frontmatter, and
+    // links stay intact so the write path is exercised exactly as in production.
+    const text = PICK.has(u.id)
+      ? u.sourceText.replace(/^(#{1,6} )(.+)$/gm, (_m, h, rest) => `${h}[DE] ${rest}`)
+      : u.sourceText;
+    entries.set(u.id, {
+      text,
+      sourceHash: "seedhash0001",
+      provenance: "machine",
+      outputHash: "0".repeat(12),
+      stale: false,
+    });
+  }
+  await adapter.write("de", entries);
+  console.log(`Seeded ${entries.size} German docs pages (${PICK.size} with [DE] prefix).`);
+}
+
+main().catch((err) => {
+  console.error(err);
+  process.exit(1);
+});