feat: show build SHA + timestamp in a small footer for deploy verification

Adds a BuildStamp component pinned to the bottom-right of every page
(desktop only, dim monospace text) so it's obvious at a glance which
build is currently running.

The git short SHA and an ISO build timestamp are injected at build time
via Vite's `define`, falling back to 'unknown' if git is not available.
ESLint config declares the two compile-time constants as readonly
globals so no per-file disable comments are needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/ui/BuildStamp.jsx

@@ -0,0 +1,19 @@
+const sha = typeof __BUILD_SHA__ === 'string' ? __BUILD_SHA__ : 'dev';
+const time = typeof __BUILD_TIME__ === 'string' ? __BUILD_TIME__ : new Date().toISOString();
+
+const formatted = (() => {
+  const d = new Date(time);
+  if (Number.isNaN(d.getTime())) return time;
+  return d.toLocaleString('nl-NL', { dateStyle: 'short', timeStyle: 'short' });
+})();
+
+const BuildStamp = () => (
+  <div
+    className="hidden md:block fixed bottom-1 right-2 text-[10px] font-mono text-fg-muted/60 z-40 pointer-events-none select-none"
+    title={`Build ${sha} at ${time}`}
+  >
+    v{sha} · {formatted}
+  </div>
+);
+
+export default BuildStamp;