fix: Settings never shows "Saved." for a URL the input no longer shows (closes #695)

The RPC and Blockscout "Save" buttons in Settings now store nothing if the input no longer shows the URL that was checked. The input loses that URL when Settings is drawn again during the check. When another page changes the network, "Use Blockscout" or "Check recipients on Etherscan", this page draws Settings again. Before this change, the typed URL was stored and "Saved." appeared while the input showed the previous URL.

The save compares the input and sets the URL in `state` in one step. A save refused because the network changed shows "The URL was not saved because the network changed." A save refused because the input changed shows "The URL was not saved because it was changed during the check."

Model: opus-5-5

README.md

@@ -2279,6 +2279,9 @@ view would leave a wallet one click from deletion.
           no user data and is made whether or not "Use Blockscout" is ticked.
     - If the network changes before either "Save" stores its URL, the URL is not
       saved and a flash message says so
+    - Either "Save" stores its URL only if the input still shows that URL when
+      the endpoint answers. Otherwise a flash message says that the URL was not
+      saved
     - Etherscan:
         - "Check recipients on Etherscan" checkbox, unticked until the user
           ticks it