fix: the first wallet's first address is active as soon as it is added (closes #512)

The popup picked an active address only when it opened, so after the first
wallet was created nothing was active until a reopen: Home had no headline,
and Send and Receive refused. Each of the three import paths on AddWallet now
makes the new wallet's first address active when no address is. The
background already answered sites with that address through its own
fallback, so nothing is broadcast. The address scan stand-in in the popup
test boot now answers in the real scan's shape, which the phrase and xprv
imports read.

Model: opus-5-5

README.md

@@ -1488,8 +1488,10 @@ view would leave a wallet one click from deletion.
 - **Transitions**:
     - "Import" with a valid entry and a matching password of at least 12
       characters → creates the wallet, clears the navigation stack, and →
-      **Home**. The phrase and xprv modes then scan for further used addresses
-      and report the count as a flash message.
+      **Home**. When no address is active yet, as on the first wallet, the new
+      wallet's first address becomes the active one. The phrase and xprv modes
+      then scan for further used addresses and report the count as a flash
+      message.
     - "Import" with a missing, short or mismatched password → full-sentence
       error on the error line, no screen change
     - "Import" with an invalid entry or a duplicate wallet or address → flash