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/docs/.vitepress/theme/MachineTranslationBanner.vue

@@ -0,0 +1,64 @@
+<script setup lang="ts">
+import { useData } from "vitepress";
+import { computed, ref } from "vue";
+import { normalizeLocale, t } from "../i18n/ui.mjs";
+
+const { lang } = useData();
+// Guard both existence AND callability: VitePress SSR defines a `localStorage`
+// global that is not a real Storage, so `getItem` may not be a function there.
+function readDismissed() {
+  try {
+    return (
+      typeof localStorage?.getItem === "function" && localStorage.getItem("so-mt-banner") === "1"
+    );
+  } catch {
+    return false;
+  }
+}
+const dismissed = ref(readDismissed());
+const locale = computed(() => normalizeLocale(lang.value));
+const show = computed(() => locale.value !== "en" && !dismissed.value);
+const text = computed(() => t(locale.value, "banner.text"));
+const cta = computed(() => t(locale.value, "banner.cta"));
+
+function dismiss() {
+  dismissed.value = true;
+  try {
+    localStorage.setItem("so-mt-banner", "1");
+  } catch {
+    // ignore storage failures (private mode)
+  }
+}
+</script>
+
+<template>
+  <div v-if="show" class="mt-banner" role="note">
+    <span>{{ text }}</span>
+    <a href="https://github.com/snapotter-hq/snapotter/blob/main/apps/docs/guide/translations.md">{{ cta }}</a>
+    <button type="button" class="mt-dismiss" aria-label="Dismiss" @click="dismiss">×</button>
+  </div>
+</template>
+
+<style scoped>
+.mt-banner {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  padding: 8px 16px;
+  font-size: 13px;
+  background: #FFF5ED;
+  border-bottom: 1px solid #EBDDCC;
+  color: #7A4A1E;
+}
+.mt-banner a { color: #A85518; font-weight: 600; }
+.mt-dismiss {
+  margin-inline-start: auto;
+  border: 0;
+  background: transparent;
+  font-size: 18px;
+  line-height: 1;
+  cursor: pointer;
+  color: inherit;
+}
+:root.dark .mt-banner { background: #241A13; border-color: #3A2C20; color: #E0C6A8; }
+</style>