fix: only the user switches the wallet's network (closes #408)

A connected site's wallet_switchEthereumChain request for the other
supported network now opens a prompt in its own window, through the
existing approval machinery, naming the site and both networks. The
network, endpoints, balances and caches change, and chainChanged is
sent, only when the user approves it; rejecting or closing the prompt
answers 4001. One such prompt per site at a time; a request for the
active network needs none. The approval window no longer shows the
connection prompt while it waits for the approval's description,
since both prompts answer on the same port.

Model: opus-5-5

README.md

@@ -1785,10 +1785,11 @@ view would leave a wallet one click from deletion.
       plus a "+ Add token" button
     - Display: "Show tracked tokens with zero balance" checkbox, "UTC
       Timestamps" checkbox, and a Theme selector (System / Light / Dark)
-    - Network: network selector (Ethereum Mainnet / Sepolia Testnet); switching
+    - Network: network selector (Ethereum Mainnet / Sepolia Testnet). The
-      resets the RPC and Blockscout endpoints to that network's defaults. The
       network changes only here, or when the user approves a site's request on
-      **NetworkApproval**
+      **NetworkApproval**; either way, switching restores the RPC and Blockscout
+      endpoints last used on that network, or that network's defaults if it has
+      none
     - Ethereum RPC: endpoint URL input + "Save" button (validated against
       `eth_chainId` before being saved)
     - Blockscout API: endpoint URL input + "Save" button (validated against
@@ -2197,8 +2198,9 @@ view would leave a wallet one click from deletion.
     - "Switch" / "Reject" buttons
 - **Transitions**:
     - "Switch" → closes popup; the background switches the network as
-      **Settings** does, sends `chainChanged` to every open tab, and answers the
+      **Settings** does, restoring the RPC and Blockscout endpoints last used on
-      site with success
+      that network (or that network's defaults if it has none), sends
+      `chainChanged` to every open tab, and answers the site with success
     - "Reject" → closes popup; the site is answered with EIP-1193 code 4001 and
       nothing changes
     - Popup window closed without answering → the same as "Reject"