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out of date, so it is now one constant.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.

dist/app.js

@@ -3589,7 +3589,7 @@ var EnhancedSecureP2PChat = () => {
       } catch (error) {
       }
     }
-    handleMessage(" SecureBit.chat Enhanced Security Edition v5.5.2 - ECDH + DTLS + SAS initialized. Ready to establish a secure connection with ECDH key exchange, DTLS fingerprint verification, and SAS authentication to prevent MITM attacks.", "system");
+    handleMessage(" SecureBit.chat Enhanced Security Edition v5.5.3 - ECDH + DTLS + SAS initialized. Ready to establish a secure connection with ECDH key exchange, DTLS fingerprint verification, and SAS authentication to prevent MITM attacks.", "system");
     manager.setFileTransferCallbacks(
       // Progress callback — drives the voice-note upload/download ring.
       (progress) => {