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.

meta.json

@@ -1,10 +1,10 @@
 {
-  "version": "1785141222839",
-  "buildVersion": "1785141222839",
-  "appVersion": "5.5.3",
-  "buildTime": "2026-07-27T08:33:42.891Z",
-  "buildId": "1785141222839-6152a77",
-  "gitHash": "6152a77",
+  "version": "1785142410213",
+  "buildVersion": "1785142410213",
+  "appVersion": "5.5.4",
+  "buildTime": "2026-07-27T08:53:30.254Z",
+  "buildId": "1785142410213-fcee421",
+  "gitHash": "fcee421",
   "generated": true,
-  "generatedAt": "2026-07-27T08:33:42.893Z"
+  "generatedAt": "2026-07-27T08:53:30.255Z"
 }