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

uk/manifest.json

@@ -0,0 +1,121 @@
+{
+  "name": "SecureBit.chat - приватний месенджер із наскрізним шифруванням",
+  "short_name": "SecureBit",
+  "description": "P2P-месенджер із захистом ECDH, DTLS і SAS та криптографією військового рівня",
+  "start_url": "/uk/",
+  "display": "standalone",
+  "background_color": "#1a1a1a",
+  "theme_color": "#ff6b35",
+  "orientation": "portrait-primary",
+  "scope": "/",
+  "lang": "uk",
+  "dir": "ltr",
+  "categories": [
+    "communication",
+    "security",
+    "productivity"
+  ],
+  "prefer_related_applications": false,
+  "icons": [
+    {
+      "src": "/logo/icon-72x72.png",
+      "sizes": "72x72",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/logo/icon-96x96.png",
+      "sizes": "96x96",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/logo/icon-128x128.png",
+      "sizes": "128x128",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/logo/icon-144x144.png",
+      "sizes": "144x144",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/logo/icon-152x152.png",
+      "sizes": "152x152",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/logo/icon-180x180.png",
+      "sizes": "180x180",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/logo/icon-192x192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/logo/icon-256x256.png",
+      "sizes": "256x256",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/logo/icon-384x384.png",
+      "sizes": "384x384",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/logo/icon-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/logo/icon-192x192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "maskable"
+    },
+    {
+      "src": "/logo/icon-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ],
+  "shortcuts": [
+    {
+      "name": "Create Channel",
+      "short_name": "Create",
+      "description": "Create a new secure channel",
+      "url": "/uk/?action=create",
+      "icons": [
+        {
+          "src": "/logo/icon-96x96.png",
+          "sizes": "96x96",
+          "type": "image/png"
+        }
+      ]
+    },
+    {
+      "name": "Join Channel",
+      "short_name": "Join",
+      "description": "Join an existing secure channel",
+      "url": "/uk/?action=join",
+      "icons": [
+        {
+          "src": "/logo/icon-96x96.png",
+          "sizes": "96x96",
+          "type": "image/png"
+        }
+      ]
+    }
+  ]
+}