feat(i18n): nine languages, each at its own address; release v6.3.0

The site now speaks German, French, Spanish, Ukrainian, Russian, Chinese,
Korean and Hindi alongside English — 623 strings per language, 5,607
translations, covering the landing page, the key exchange, the chat, group
calls and every error along the way.

Each locale is a real page at a real URL (/de/, /fr/, …), generated at build
time from locales/*.json. That is the whole point: the app is client-rendered,
so a language that only swaps strings at runtime has no address for a crawler
to index and no link anyone can share. Each page carries its own canonical, a
reciprocal hreflang cluster and translated schema.org.

The URL decides the language, always. A stored preference only applies where
the address does not say, and nothing ever redirects on Accept-Language —
that is how sites become invisible to search engines outside one country.

robots.txt and sitemap.xml did not exist before; both are generated now.

Bugs found on the way, each with a test that would have caught it:
  - partner logos used page-relative paths and 404'd from any /xx/ page
  - post-build stamped ?v= only into the root shell, leaving locales behind
  - "Back online" appeared on every page load, not just after being offline
  - update timestamps were hard-coded to US format for every reader
  - t() threw on a partial window, taking whole components down with it

meta.json

@@ -1,10 +1,10 @@
 {
-  "version": "1787944850227",
-  "buildVersion": "1787944850227",
-  "appVersion": "6.2.0",
-  "buildTime": "2026-08-28T19:20:50.271Z",
-  "buildId": "1787944850227-e00c3bd",
-  "gitHash": "e00c3bd",
+  "version": "1788021796445",
+  "buildVersion": "1788021796445",
+  "appVersion": "6.3.0",
+  "buildTime": "2026-08-29T16:43:16.486Z",
+  "buildId": "1788021796445-943e04e",
+  "gitHash": "943e04e",
   "generated": true,
-  "generatedAt": "2026-08-28T19:20:50.272Z"
+  "generatedAt": "2026-08-29T16:43:16.487Z"
 }