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

deploy/nginx.conf

@@ -54,6 +54,33 @@ http {
         # (e.g. qr-local.js ~1.2MB) when nothing changed, while still picking up
         # new releases immediately.
         ~^/dist/                 "no-cache, must-revalidate";
+        # The install prompt is a plain browser module and imports the dictionary by a
+        # bare path, so that import carries no ?v= to bust the cache with. Left on the
+        # default one-year immutable rule below, a visitor would keep the translations
+        # they first loaded for a year.
+        ~^/src/i18n/             "no-cache, must-revalidate";
+        # Crawler-facing files must stay re-fetchable: the default below is a
+        # one-year immutable cache, which would freeze a sitemap for a year.
+        ~^/robots\.txt$          "public, max-age=3600";
+        ~^/sitemap\.xml$         "public, max-age=3600";
+    }
+
+    # Which app shell an unmatched navigation falls back to. A request under /de/ must
+    # land on /de/index.html, not the English one — otherwise every deep link into a
+    # localized page silently switches language. The entries between the markers are
+    # rewritten by scripts/build-i18n.js from locales/site.json.
+    map $uri $sb_shell {
+        default /index.html;
+        # BEGIN generated locale shells
+        ~^/de/  /de/index.html;
+        ~^/fr/  /fr/index.html;
+        ~^/es/  /es/index.html;
+        ~^/uk/  /uk/index.html;
+        ~^/ru/  /ru/index.html;
+        ~^/zh/  /zh/index.html;
+        ~^/ko/  /ko/index.html;
+        ~^/hi/  /hi/index.html;
+        # END generated locale shells
     }
 
     # CDN-Cache-Control is read by Cloudflare (and other CDNs) *independently* of the
@@ -71,6 +98,7 @@ http {
         ~^/meta\.json$           "no-store";
         ~^/config/ice-servers\.js$ "no-store";
         ~^/dist/                 "no-store";
+        ~^/src/i18n/             "no-store";
     }
 
     server {
@@ -99,13 +127,13 @@ http {
         # Real asset files must return 404 when missing — never fall back to the
         # HTML shell, which would be served with the wrong content type and break
         # module/script loading (e.g. a missing config/ice-servers.js).
-        location ~* \.(js|mjs|jsx|css|json|map|woff2?|ttf|otf|png|jpe?g|gif|webp|svg|ico|mp3|mp4|webm)$ {
+        location ~* \.(js|mjs|jsx|css|json|map|xml|txt|woff2?|ttf|otf|png|jpe?g|gif|webp|svg|ico|mp3|mp4|webm)$ {
             try_files $uri =404;
         }
 
         # SPA-style fallback so unknown navigation routes still load the app shell.
         location / {
-            try_files $uri $uri/ /index.html;
+            try_files $uri $uri/ $sb_shell;
         }
     }
 }