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

.htaccess

@@ -104,6 +104,15 @@
     </IfModule>
 </FilesMatch>
 
+# The dictionary is imported by a bare path from a plain browser module, so nothing
+# busts its cache. It must revalidate or translations freeze at whatever a visitor
+# first loaded.
+<FilesMatch "^(index|generated)\.js$">
+    <IfModule mod_headers.c>
+        Header set Cache-Control "no-cache, must-revalidate"
+    </IfModule>
+</FilesMatch>
+
 # CSS files - long cache
 <FilesMatch "\.css$">
     <IfModule mod_headers.c>
@@ -133,6 +142,13 @@
     </IfModule>
 </FilesMatch>
 
+# Crawler-facing files - short cache so search engines re-read them
+<FilesMatch "^(robots\.txt|sitemap\.xml)$">
+    <IfModule mod_headers.c>
+        Header set Cache-Control "public, max-age=3600"
+    </IfModule>
+</FilesMatch>
+
 # ============================================
 # SECURITY
 # ============================================
@@ -186,6 +202,35 @@
 
 # If file not found, redirect to index.html (for SPA routing)
 <IfModule mod_rewrite.c>
+    # Locale shells first: a deep link under /de/ must fall back to the German page,
+    # not the English one. Rewritten by scripts/build-i18n.js from locales/site.json.
+    # BEGIN generated locale shells
+    RewriteCond %{REQUEST_FILENAME} !-f
+    RewriteCond %{REQUEST_FILENAME} !-d
+    RewriteRule ^de(/.*)?$ /de/index.html [L]
+    RewriteCond %{REQUEST_FILENAME} !-f
+    RewriteCond %{REQUEST_FILENAME} !-d
+    RewriteRule ^fr(/.*)?$ /fr/index.html [L]
+    RewriteCond %{REQUEST_FILENAME} !-f
+    RewriteCond %{REQUEST_FILENAME} !-d
+    RewriteRule ^es(/.*)?$ /es/index.html [L]
+    RewriteCond %{REQUEST_FILENAME} !-f
+    RewriteCond %{REQUEST_FILENAME} !-d
+    RewriteRule ^uk(/.*)?$ /uk/index.html [L]
+    RewriteCond %{REQUEST_FILENAME} !-f
+    RewriteCond %{REQUEST_FILENAME} !-d
+    RewriteRule ^ru(/.*)?$ /ru/index.html [L]
+    RewriteCond %{REQUEST_FILENAME} !-f
+    RewriteCond %{REQUEST_FILENAME} !-d
+    RewriteRule ^zh(/.*)?$ /zh/index.html [L]
+    RewriteCond %{REQUEST_FILENAME} !-f
+    RewriteCond %{REQUEST_FILENAME} !-d
+    RewriteRule ^ko(/.*)?$ /ko/index.html [L]
+    RewriteCond %{REQUEST_FILENAME} !-f
+    RewriteCond %{REQUEST_FILENAME} !-d
+    RewriteRule ^hi(/.*)?$ /hi/index.html [L]
+    # END generated locale shells
+
     RewriteCond %{REQUEST_FILENAME} !-f
     RewriteCond %{REQUEST_FILENAME} !-d
     RewriteCond %{REQUEST_URI} !^/meta\.json$