fix(tokens): disable Convert on Dollars when nothing else is held

[bmc08gt]

A Dollars-only user could open Convert on the Dollars screen into a flow with no destination. Bugsnag 6ac36f88991833fd09434aad: the Convert-from-Dollars fallback picked a stale zero-balance USDC row as the destination, and every confirm failed with "mint not found". Filtering that row out leaves the destination on Dollars, where tapping confirm on the receipt spins forever because ProceedWithConversion returned early without resetting loading.

Change

• The fallback for a Convert opened from Dollars only picks held currencies with a displayable balance, the rule the destination picker applies. TokenCoordinator keeps zero-balance rows for accounts it no longer sees, and the server rejects a buy into one. This was fix(swap): don't default a Convert from Dollars to an empty holding #1677, folded in here.
• The Convert tile on the Dollars screen is disabled, not hidden, unless the user holds another currency with a displayable balance. No hint text or toast.
• TokenInfoViewModel.State.canConvert reuses the existing fundableBalanceMints set, which is built with the same hasDisplayableValue filter as the default-destination fallback. The set is empty until tokenBalances hydrates, so Convert starts disabled while balances load.
• Other token screens are unchanged: Dollars is always a destination from them, so canConvert is true.
• OnConvert also checks canConvert, so nothing else can open the flow with no destination.
• ProceedWithConversion clears loading on every early return, as a backstop if a destination disappears between the button and the receipt.

The destination picker applies the same rule to the localized amount (nativeAmount.hasDisplayableValue), so the two can differ only on dust. Neither filters on launchpadMetadata, so the button doesn't either.

Tests: TokenInfoViewModelConvertTest covers loading, Dollars-only, a zero-balance other holding, another holding with a balance (opens Convert), and a non-Dollars screen. SwapViewModelErrorTest covers the fallback (largest other holding, never an empty one) and the loading reset.