fix: Settings refuses an endpoint URL whose network changed before it was stored (closes #663)

The RPC and Blockscout "Save" buttons in Settings now set the URL
inside the save, holding the lock. The save is refused if the network
active when the user tapped "Save" is no longer active, on the page or
in the stored record. The flash message then says that the network
changed, and "Saved." is shown only for a stored URL.

Before, a popup that followed a switch saved elsewhere could replace
the URL before the write and still show "Saved.". It could also write
the previous network's URL as the new network's.

Model: opus-5-5

README.md

@@ -2281,6 +2281,8 @@ view would leave a wallet one click from deletion.
         - Endpoint URL input + "Save" button. "Save" asks the URL for `/stats`
           and saves the URL only if that request succeeds. The request carries
           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
     - Etherscan:
         - "Check recipients on Etherscan" checkbox, unticked until the user
           ticks it