harden: one connection and one signature prompt per site at a time (closes #405)

Each eth_requestAccounts or personal_sign call opened another approval
window, so a page calling in a loop could cover the screen with identical
prompts. While a site's connection or signature prompt is unanswered, a
further request of that kind from the same site is now refused with
EIP-1193 -32002, the code a second transaction already gets, and opens no
window. The pending approval itself is the record: the check and the
approval's creation have nothing awaited between them. Other sites are
unaffected, and the site may ask again once the user answers.

Model: opus-5-5

README.md

@@ -1840,7 +1840,9 @@ view would leave a wallet one click from deletion.
   `wallet_requestPermissions` and is on neither the allowed nor the denied list.
   The background script prefers the toolbar popup (`action.openPopup()`) and
   falls back to a separate popup window (`src/background/index.js`,
-  `requestApproval()`).
+  `requestApproval()`). Only one exists per site at a time: a further connection
+  request from a site whose prompt is still unanswered is refused with EIP-1193
+  code `-32002` and opens nothing.
 - **Elements**:
     - "Connection Request" heading
     - Phishing warning banner (shown when the hostname is on the phishing
@@ -1901,7 +1903,10 @@ view would leave a wallet one click from deletion.
 
 - **When**: A connected website requests a message signature via
   `personal_sign`, `eth_sign`, or `eth_signTypedData_v4`. Opened the same way as
-  TxApproval, in a separate popup window.
+  TxApproval, in a separate popup window. Only one exists per site at a time: a
+  further signature request, by any of these methods, from a site whose
+  signature request is still unanswered is refused with EIP-1193 code `-32002`
+  and opens no window.
 - **Elements**:
     - "Signature Request" heading
     - Phishing warning banner (shown when the hostname is on the phishing