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. When the
phrase or xprv scan then replaces the address list with one that leaves that
address out, the new list's first address becomes active and the background
is told, as when the user picks an address on Home. 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

@@ -1552,8 +1552,14 @@ 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. When the scan finds two or more used addresses, they replace the
+      wallet's list, which then holds the first address only if it was used. If
+      the first address was active and is not in the new list, the new list's
+      first address becomes the active one and `AUTISTMASK_ACTIVE_CHANGED` is
+      broadcast.
     - "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