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

manifest.json

@@ -1,5 +1,5 @@
 {
-  "name": "SecureBit.chat — Private, Encrypted Messenger",
+  "name": "SecureBit.chat - Private, Encrypted Messenger",
   "short_name": "SecureBit",
   "description": "P2P messenger with ECDH + DTLS + SAS security, military-grade cryptography and Lightning Network payments",
   "start_url": "./",
@@ -10,7 +10,11 @@
   "scope": "./",
   "lang": "en",
   "dir": "ltr",
-  "categories": ["communication", "security", "productivity"],
+  "categories": [
+    "communication",
+    "security",
+    "productivity"
+  ],
   "prefer_related_applications": false,
   "icons": [
     {
@@ -102,7 +106,7 @@
     },
     {
       "name": "Join Channel",
-      "short_name": "Join", 
+      "short_name": "Join",
       "description": "Join an existing secure channel",
       "url": "./?action=join",
       "icons": [