diff --git a/index.html b/index.html
index 4330cb0..823f990 100644
--- a/index.html
+++ b/index.html
@@ -39,29 +39,8 @@
       href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&family=JetBrains+Mono:wght@400;500&family=Sora:wght@600;700&display=swap"
     />
 
-    <!-- Apply the saved/system theme before first paint to avoid a light flash. -->
-    <script src="/theme-init.js"></script>
     <!-- Apply the saved/system theme and language before first paint to avoid a flash. -->
-    <script>
-      (function () {
-        try {
-          var stored = localStorage.getItem("synapsvault-theme");
-          var dark = stored ? stored === "dark" : window.matchMedia("(prefers-color-scheme: dark)").matches;
-          if (dark) document.documentElement.classList.add("dark");
-        } catch (e) {}
-        try {
-          var storedLang = localStorage.getItem("synapsvault-lang");
-          var navLang = (navigator.language || "").split("-")[0].toLowerCase();
-          var lang =
-            storedLang === "en" || storedLang === "es"
-              ? storedLang
-              : navLang === "es"
-                ? "es"
-                : "en";
-          document.documentElement.lang = lang;
-        } catch (e) {}
-      })();
-    </script>
+    <script src="/theme-init.js"></script>
   </head>
   <body>
     <div id="root"></div>
diff --git a/public/theme-init.js b/public/theme-init.js
index 891e198..1f5fbb6 100644
--- a/public/theme-init.js
+++ b/public/theme-init.js
@@ -1,3 +1,4 @@
+// Apply the saved theme before first paint to avoid a flash.
 (function () {
   try {
     var stored = localStorage.getItem("synapsvault-theme");
@@ -5,3 +6,18 @@
     if (dark) document.documentElement.classList.add("dark");
   } catch (e) {}
 })();
+
+// Apply the saved language before first paint to avoid a flash.
+(function () {
+  try {
+    var storedLang = localStorage.getItem("synapsvault-lang");
+    var navLang = (navigator.language || "").split("-")[0].toLowerCase();
+    var lang =
+      storedLang === "en" || storedLang === "es"
+        ? storedLang
+        : navLang === "es"
+          ? "es"
+          : "en";
+    document.documentElement.lang = lang;
+  } catch (e) {}
+})();
diff --git a/src/api/requestSignature.ts b/src/api/requestSignature.ts
index 213a1da..e7de419 100644
--- a/src/api/requestSignature.ts
+++ b/src/api/requestSignature.ts
@@ -2,10 +2,23 @@
  * HMAC-SHA256 request signing for publisher mutations.
  * Mirrors server/src/utils/requestSignature.ts — keep in sync when changing the scheme.
  *
- * Replay protection: the signature covers a Unix-seconds timestamp but no nonce, so a
- * captured signed request can be replayed for as long as the server accepts that timestamp.
- * Replay defence must be enforced server-side: bound the accepted timestamp window and reject
- * repeated Idempotency-Keys / signatures within it. Mutations should send a unique Idempotency-Key.
+ * Replay protection: the signature covers a Unix-seconds timestamp but no nonce. A captured
+ * signed request can be replayed as long as the server accepts that timestamp. The server
+ * MUST implement replay defense:
+ *
+ * 1. **Timestamp window**: Enforce a reasonable window (e.g., ±5 minutes from server time).
+ *    Requests outside this window are rejected, limiting the replay attack window.
+ *
+ * 2. **Idempotency-Key deduplication**: Track recent Idempotency-Keys per API key and reject
+ *    replayed signatures within the timestamp window. Combined with the timestamp check, this
+ *    prevents the same signed request from being submitted twice.
+ *
+ * 3. **Nonce (optional)**: For higher security, the server may require a server-provided nonce
+ *    in the signature and invalidate used nonces. This prevents even timestamp-replayed requests
+ *    outside the window. Nonces are not yet implemented in this client but are compatible with
+ *    this signing scheme (add nonce to the canonical string before signing).
+ *
+ * Client responsibility: Always provide a unique Idempotency-Key for each mutation.
  */
 
 import { networkFetch } from "./http.js";
@@ -105,15 +118,13 @@ export async function signedPublisherFetch(
 ): Promise<Response> {
   const method = (init.method ?? "GET").toUpperCase();
   const body = typeof init.body === "string" ? init.body : undefined;
-  const path = new URL(url).pathname + new URL(url).search;
-  const requestHeaders = new Headers(init.headers);
-  const idempotencyKey = requestHeaders.get("Idempotency-Key") ?? undefined;
   const parsed = new URL(url, typeof window !== "undefined" ? window.location.origin : "http://localhost");
   const path = parsed.pathname + parsed.search;
-  const idempotencyKey =
-    init.headers instanceof Headers
-      ? (init.headers.get("Idempotency-Key") ?? undefined)
-      : (init.headers as Record<string, string> | undefined)?.["Idempotency-Key"];
+
+  // Normalize headers to ensure array-form headers are properly read.
+  // Without normalization, Idempotency-Key from array-form headers would be missed.
+  const requestHeaders = new Headers(init.headers);
+  const idempotencyKey = requestHeaders.get("Idempotency-Key") ?? undefined;
 
   const signatureHeaders = await signPublisherRequestHeaders({
     apiKey,
diff --git a/vercel.json b/vercel.json
index 3bafb2b..b02ffb7 100644
--- a/vercel.json
+++ b/vercel.json
@@ -28,6 +28,10 @@
         {
           "key": "Content-Security-Policy",
           "value": "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; font-src 'self' https://fonts.gstatic.com; img-src 'self' data: blob:; connect-src 'self' https://horizon.stellar.org https://horizon-testnet.stellar.org https://soroban.stellar.org https://soroban-testnet.stellar.org https://*.ingest.sentry.io; object-src 'none'; base-uri 'self'; frame-ancestors 'self'; form-action 'self'"
+        },
+        {
+          "key": "Permissions-Policy",
+          "value": "camera=(), microphone=(), geolocation=(), usb=()"
         }
       ]
     },