fix(swap): don't default a Convert from Dollars to an empty holding

[bmc08gt]

Converting from Dollars opens with Dollars as the destination, and SwapViewModel swaps in the largest other entry in tokenBalances. That list can hold zero-balance rows, because TokenState.withAccounts only ever adds holdings. In Bugsnag 6ac36f88991833fd09434aad (2026.9.4), a user holding only Dollars got a leftover USDC row as the destination. All six attempts logged buy of USDC and failed with INVALID_SWAP: mint not found. The first three came before they opened the destination picker, so the fallback chose USDC, not the user.

The fallback now only considers held currencies with a displayable balance. The destination picker (SelectTokenViewModel) and iOS (Session.balances(for:)) already use that filter. With no candidate, the destination stays on Dollars.

Two cases added to SwapViewModelErrorTest: the fallback still picks the largest real holding, and an empty row is never picked.

Not covered here:

• A Dollars-only holder still has no valid destination. Confirm likely leaves the spinner on, because ProceedWithConversion sets loading before bailing. What that user should see is still open.
• withAccounts never prunes holdings, so stale rows still reach every tokenBalances reader.

[bmc08gt]

Folded into #1679, which carries this commit along with the disabled Convert button.