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.

dist/app-boot.js

@@ -18910,7 +18910,7 @@ Right-click or Ctrl+click to disconnect`,
           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")
           ])

dist/app.js

@@ -1805,10 +1805,12 @@ var EnhancedConnectionSetup = ({
       }, disabled: !hasInvite || connectionStatus === "connecting", style: { width: "100%", display: "inline-flex", alignItems: "center", justifyContent: "center", gap: "9px", padding: "14px", borderRadius: "13px", border: "none", background: hasInvite && connectionStatus !== "connecting" ? C_ORANGE : "rgba(255,255,255,0.05)", color: hasInvite && connectionStatus !== "connecting" ? "#1a0f04" : "#56565e", fontFamily: "inherit", fontSize: "15px", fontWeight: 700, cursor: hasInvite && connectionStatus !== "connecting" ? "pointer" : "not-allowed", boxShadow: hasInvite && connectionStatus !== "connecting" ? "0 8px 24px rgba(240,137,42,0.28)" : "none" } }, connectionStatus === "connecting" ? "Processing\u2026" : "Connect")
     ]);
   }
+  const SB_DESKTOP_VERSION = "0.3.0";
+  const SB_DESKTOP_RELEASE = `https://github.com/SecureBitChat/securebit-desktop/releases/download/v${SB_DESKTOP_VERSION}`;
   const DOWNLOADS = {
-    mac: { name: "macOS", format: ".dmg \xB7 Apple Silicon & Intel", icon: "fab fa-apple", url: "https://github.com/SecureBitChat/securebit-desktop/releases/download/v0.1.0/SecureBit.Chat_0.1.0_x64.dmg" },
-    win: { name: "Windows", format: ".exe \xB7 64-bit installer", icon: "fab fa-windows", url: "https://github.com/SecureBitChat/securebit-desktop/releases/latest/download/SecureBit.Chat_0.1.0_x64-setup.exe" },
-    linux: { name: "Linux", format: ".AppImage", icon: "fab fa-linux", url: "https://github.com/SecureBitChat/securebit-desktop/releases/latest/download/SecureBit.Chat_0.1.0_amd64.AppImage" }
+    mac: { name: "macOS", format: ".dmg \xB7 Apple Silicon & Intel", icon: "fab fa-apple", url: `${SB_DESKTOP_RELEASE}/SecureBit.Chat_${SB_DESKTOP_VERSION}_x64.dmg` },
+    win: { name: "Windows", format: ".exe \xB7 64-bit installer", icon: "fab fa-windows", url: `${SB_DESKTOP_RELEASE}/SecureBit.Chat_${SB_DESKTOP_VERSION}_x64-setup.exe` },
+    linux: { name: "Linux", format: ".AppImage", icon: "fab fa-linux", url: `${SB_DESKTOP_RELEASE}/SecureBit.Chat_${SB_DESKTOP_VERSION}_amd64.AppImage` }
   };
   const detectOS = () => {
     const ua = (navigator.userAgent || "") + " " + (navigator.platform || "");
@@ -3589,7 +3591,7 @@ var EnhancedSecureP2PChat = () => {
       } catch (error) {
       }
     }
-    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");
+    handleMessage(" SecureBit.chat Enhanced Security Edition v5.5.4 - 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) => {