[Wallet] Bugfix/enable-funding-multiple-mints (#210)

* Fix bug where no additional mints can be funded

As soon as one mint is in the database, the new multimint selector only
considers this mint, even if the `MINT_` environment variables point to
a different one. This commit fixes it by explicitly adding the set mint
URL to the selector.

Additionally did some small cleanups.

* Move mint adding to higher-level function

* bypass selection at first time

* use contextx, more explicit

* remove whitespace

---------

Co-authored-by: xphade <18196286+xphade@users.noreply.github.com>

cashu/wallet/cli/cli_helpers.py

@@ -30,8 +30,10 @@ async def get_mint_wallet(ctx: Context):
     wallet: Wallet = ctx.obj["WALLET"]
     mint_balances = await wallet.balance_per_minturl()
 
-    # if we have balances on more than one mint, we ask the user to select one
-    if len(mint_balances) > 1:
+    if ctx.obj["HOST"] not in mint_balances:
+        mint_url = wallet.url
+    elif len(mint_balances) > 1:
+        # if we have balances on more than one mint, we ask the user to select one
         await print_mint_balances(wallet, show_mints=True)
 
         url_max = max(mint_balances, key=lambda v: mint_balances[v]["available"])