Point desktop downloads at 0.3.0; release v5.5.3

The Download block still offered desktop 0.1.0 — the build from before in-app
updates, the voice-note fixes and the verification hardening. Windows, macOS and
Linux now link to 0.3.0.

The version was repeated across three URLs in two different forms: two resolved
through /releases/latest/download/ and one pinned a tag. That is how they drifted
out of date, so it is now one constant.

The tag stays pinned on purpose. Release filenames carry the version, so a
/latest/ link breaks the moment a newer release exists, while a pinned tag keeps
serving a working installer — the safer way to fail if the constant is ever left
behind.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "securebit-chat",
-  "version": "5.5.2",
+  "version": "5.5.3",
   "description": "Secure P2P Communication Application with End-to-End Encryption",
   "main": "index.html",
   "scripts": {