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

The RPC and Blockscout "Save" buttons in Settings take the network that
Settings was last drawn for. A follow of a switch saved elsewhere sets
`state` first. Settings is drawn again only once the follow's write
returns.

The URL is set in `state` inside the save, holding the lock. The save is
refused if the network that Settings showed is no longer active, on the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 store a URL
for a network that Settings did not show.

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