feat(welcome): link to the public source repository from the footer

Siftlode is open source; add a discreet 'Source code' link (trilingual) in the
landing-page footer next to Privacy/Terms, pointing at the public repo.

frontend/src/components/Welcome.tsx

@@ -1,6 +1,7 @@
 import { useEffect, useState } from "react";
 import { useTranslation } from "react-i18next";
 import {
+  Code2,
   Expand,
   Filter,
   Inbox,
@@ -11,6 +12,9 @@ import {
   X,
   type LucideIcon,
 } from "lucide-react";
+
+// Siftlode is open source; the landing footer links to the public repository.
+const REPO_URL = "https://forge.b1fr0st.eu/peter/siftlode";
 import { api } from "../lib/api";
 import { HttpError } from "../lib/api";
 import { setLanguage, type LangCode } from "../i18n";
@@ -102,6 +106,15 @@ export default function Welcome() {
         <a href="/terms" className="hover:text-fg transition">
           {t("common.termsOfService")}
         </a>
+        <a
+          href={REPO_URL}
+          target="_blank"
+          rel="noopener noreferrer"
+          className="inline-flex items-center gap-1.5 hover:text-fg transition"
+        >
+          <Code2 className="w-3.5 h-3.5" />
+          {t("common.sourceCode")}
+        </a>
       </footer>
     </div>
   );