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.

scripts/i18n/lib/slugify.mjs

@@ -0,0 +1,27 @@
+// scripts/i18n/lib/slugify.mjs
+
+// Exact port of the slugify VitePress uses (mdit-vue @mdit-vue/shared). Keeping
+// this identical means the {#anchor} we inject equals the auto-slug VitePress
+// would produce, so pre-existing #deep-links keep resolving after we add
+// explicit anchors. Regexes copied verbatim from vitepress/dist/node, with
+// explicit unicode escapes for the control and combining ranges.
+// biome-ignore lint/suspicious/noControlCharactersInRegex: byte-exact port of mdit-vue slugify; the control-char range is required for VitePress anchor parity.
+const rControl = /[\u0000-\u001f]/g;
+const rSpecial = /[\s~`!@#$%^&*()\-_+=[\]{}|\\;:"'“”‘’<>,.?/]+/g;
+const rCombining = /[\u0300-\u036f]/g;
+
+/**
+ * @param {string} str
+ * @returns {string}
+ */
+export function slugify(str) {
+  return String(str)
+    .normalize("NFKD")
+    .replace(rCombining, "")
+    .replace(rControl, "")
+    .replace(rSpecial, "-")
+    .replace(/-{2,}/g, "-")
+    .replace(/^-+|-+$/g, "")
+    .replace(/^(\d)/, "_$1")
+    .toLowerCase();
+}