Add QR code tracker to admin panel

Each QR code gets a /qr/<slug> redirect that logs scans (IP, user agent,
timestamp) to disk. The admin QR Codes view lets you add, edit, enable/disable,
and delete codes, with per-code scan history inline.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/state.js

@@ -59,6 +59,12 @@ export const state = {
   downloadCounts: {},
   downloadCountsWritePromise: Promise.resolve(),
 
+  // qrCodes: Array<{ id, slug, label, destination, createdAt }>
+  // qrScans: Array<{ id, qrId, slug, scannedAt, ip, userAgent }>
+  qrCodes: [],
+  qrScans: [],
+  qrCodesWritePromise: Promise.resolve(),
+
   lastBackupStatus: { ok: true, at: null, error: null, file: null },
   lastCachePurgeStatus: { ok: true, at: null, error: null },
   lastDeployHookStatus: { ok: true, at: null, error: null },