fix: zero-tx warning layout shift and contract address false positive

- Reserve space for the warning upfront using visibility:hidden instead
  of display:none, preventing layout shift per README policy
- Move warning HTML to index.html as a static element rather than
  injecting dynamically
- Skip warning for contract addresses (check getCode first) since
  getTransactionCount only returns outgoing tx nonce
- Collapse reserved space when warning is not needed (address has
  history, is a contract, or on RPC error)

src/popup/index.html

@@ -577,6 +577,19 @@
                     <div id="confirm-fee-amount" class="text-xs"></div>
                 </div>
                 <div id="confirm-warnings" class="mb-2 hidden"></div>
+                <div
+                    id="confirm-recipient-warning"
+                    class="mb-2"
+                    style="visibility: hidden"
+                >
+                    <div
+                        class="border border-red-500 border-dashed p-2 text-xs font-bold text-red-500"
+                    >
+                        WARNING: The recipient address has ZERO transaction
+                        history. This may indicate a fresh or unused address.
+                        Double-check the address before sending.
+                    </div>
+                </div>
                 <div
                     id="confirm-errors"
                     class="mb-2 border border-border border-dashed p-2 hidden"