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 and opens no window; all signing methods count as one
kind. A connection prompt whose toolbar popup closed before it connected,
and which the toolbar popup no longer opens, is shown again by the site's
next request instead of refusing the site until the address changes.

Model: opus-5-5

README.md

@@ -1844,7 +1844,11 @@ view would leave a wallet one click from deletion.
   says nothing about `http://dapp.example` or another port of that host. 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 no new prompt. If that prompt was in a toolbar popup
+  that closed before it connected, and the toolbar popup has since been set to
+  open something else, the refused request shows that prompt again.
 - **Elements**:
     - "Connection Request" heading
     - Phishing warning banner (shown when the hostname is on the phishing
@@ -1910,7 +1914,10 @@ 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