fix: repair wallet state on delete (closes #156)

Deleting a wallet left three defects in the same path:

- `hasWallet` stayed true after the last wallet was deleted, so the next
  popup open rendered Home with "No wallets yet" instead of Welcome. It
  now tracks whether any wallet remains.
- The selection was reset to wallet 0 / address 0 unconditionally.
  `selectedWallet` now follows the splice (decremented when a wallet
  before it is removed), and only falls back to the first remaining
  wallet's first address when the selection itself was deleted;
  `activeAddress` only moves when it belonged to the deleted wallet.
- No `AUTISTMASK_ACTIVE_CHANGED` was sent, so connected sites kept
  reporting a deleted address. The delete path now broadcasts it after
  the state is persisted, whenever the active address actually changed,
  and the background re-emits `accountsChanged` from there.

The transition moved into `src/shared/walletDelete.js` so it can be
tested without a DOM. Site-permission cleanup is unchanged and still
covers every address of an HD wallet.

README.md

@@ -980,7 +980,7 @@ Currently supported:
 
 ### Wallet Management
 
-- [ ] Delete wallet (with confirmation)
+- [x] Delete wallet (with confirmation)
 - [ ] Delete address from HD wallet (with confirmation)
 - [ ] Show wallet's recovery phrase (requires password)