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.

src/components/ui/Header.jsx

@@ -559,7 +559,7 @@ const EnhancedMinimalHeader = ({
                     React.createElement('div', { key: 'txt', style: { lineHeight: 1.2, minWidth: 0 } }, [
                         React.createElement('div', { key: 'r1', style: { display: 'flex', alignItems: 'baseline', gap: '7px' } }, [
                             React.createElement('span', { key: 'n', style: { fontSize: '16px', fontWeight: 800, letterSpacing: '-0.3px', color: '#e8e8eb' } }, 'SecureBit'),
-                            React.createElement('span', { key: 'v', style: { fontFamily: MONO, fontSize: '10px', fontWeight: 500, color: '#56565e' } }, 'v5.5.3')
+                            React.createElement('span', { key: 'v', style: { fontFamily: MONO, fontSize: '10px', fontWeight: 500, color: '#56565e' } }, 'v5.5.4')
                         ]),
                         React.createElement('div', { key: 'r2', className: 'hidden sm:block', style: { fontSize: '11px', color: '#6b6b73', fontWeight: 500 } }, 'End-to-end encrypted')
                     ])