Fix the desktop download buttons; release v5.5.4

The buttons still led to a dead GitHub page. 5.5.3 updated one of the two places
these links live — the platforms menu on the connection screen keeps its own
DOWNLOADS table, and it was missed, so it stayed on 0.1.0.

Why it looked like a working link that did nothing: the stale entries used
/releases/latest/download/<file>, and GitHub resolves `latest` by redirecting to
the newest tag. Once 0.3.0 shipped, a link written for 0.1.0 resolved to
/releases/download/v0.3.0/SecureBit.Chat_0.1.0_x64-setup.exe — a file that never
existed under that tag. The browser navigated to GitHub and downloaded nothing.

Both places now build their URLs from a DESKTOP_VERSION constant with the tag
pinned, so the version is written once per file and a link cannot silently
become invalid when a new release goes out.

Adds tests/desktop-download-links.test.mjs, which fails the build if this drifts
again: every source must derive URLs from that constant, /latest/ and hardcoded
versions in filenames are rejected, and each generated URL is fetched to prove
the asset exists. SKIP_NETWORK=1 skips the fetches offline.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "securebit-chat",
-  "version": "5.5.3",
+  "version": "5.5.4",
   "description": "Secure P2P Communication Application with End-to-End Encryption",
   "main": "index.html",
   "scripts": {
@@ -11,7 +11,7 @@
     "dev": "npm run build && python -m http.server 8000",
     "watch": "npx tailwindcss -i src/styles/tw-input.css -o assets/tailwind.css --watch",
     "serve": "npx http-server -p 8000",
-    "test": "node tests/sas-verification.test.mjs && node tests/verification-gate.test.mjs && node tests/inbound-frame-authentication.test.mjs && node tests/security-level-shape.test.mjs && node tests/file-transfer-consent.test.mjs && node tests/incoming-message-sanitization.test.mjs && node tests/outgoing-message-integrity.test.mjs && node tests/secure-chat-features.test.mjs && node tests/notification-meta-forwarding.test.mjs && node tests/file-type-allowlist.test.mjs && node tests/webrtc-privacy-mode.test.mjs && node tests/indexeddb-metadata-encryption.test.mjs && node tests/disconnect-cleanup.test.mjs && node tests/timer-lifecycle.test.mjs && node tests/file-transfer-cleanup.test.mjs && node tests/file-transfer-ui-cleanup.test.mjs && node tests/file-transfer-callback-propagation.test.mjs && node tests/debug-window-hooks.test.mjs && node tests/inbound-message-rate-limit.test.mjs && node tests/file-transfer-chunk-rate-limit.test.mjs && node tests/ice-servers-validation.test.mjs && node tests/sessions-reducer.test.mjs && node tests/webrtc-sdp.test.mjs && node tests/webrtc-video.test.mjs && node tests/webrtc-adaptation.test.mjs"
+    "test": "node tests/sas-verification.test.mjs && node tests/verification-gate.test.mjs && node tests/inbound-frame-authentication.test.mjs && node tests/security-level-shape.test.mjs && node tests/desktop-download-links.test.mjs && node tests/file-transfer-consent.test.mjs && node tests/incoming-message-sanitization.test.mjs && node tests/outgoing-message-integrity.test.mjs && node tests/secure-chat-features.test.mjs && node tests/notification-meta-forwarding.test.mjs && node tests/file-type-allowlist.test.mjs && node tests/webrtc-privacy-mode.test.mjs && node tests/indexeddb-metadata-encryption.test.mjs && node tests/disconnect-cleanup.test.mjs && node tests/timer-lifecycle.test.mjs && node tests/file-transfer-cleanup.test.mjs && node tests/file-transfer-ui-cleanup.test.mjs && node tests/file-transfer-callback-propagation.test.mjs && node tests/debug-window-hooks.test.mjs && node tests/inbound-message-rate-limit.test.mjs && node tests/file-transfer-chunk-rate-limit.test.mjs && node tests/ice-servers-validation.test.mjs && node tests/sessions-reducer.test.mjs && node tests/webrtc-sdp.test.mjs && node tests/webrtc-video.test.mjs && node tests/webrtc-adaptation.test.mjs"
   },
   "keywords": [
     "p2p",