Fiat sentinel strings ("Disabled"/"N/A") leak into editable amount inputs when no fiat rate is available

Spawned from @ajaysehwal's review note on #4625: https://github.com/ZeusLN/zeus/pull/4625#pullrequestreview-5170149054

Problem

getUnformattedAmount() returns a localized sentinel string in its amount field in all three fiat failure branches:

• no fiat currency configured while units are fiat (utils/AmountUtils.ts L147-L153, the branch with the TODO: is this the right place to catch this? comment)
• rates fetched but no rate for the selected currency (L158-L164)
• no rates at all (L186-L192)

Because the sentinel ("Disabled" in English) is truthy, callers that do getUnformattedAmount(...).amount || fallback never hit their fallback. With #4625 this pattern is centralized in getRawAmountFromSats(), which feeds the editable amount state in views/Send.tsx and views/ClinkPay/ClinkPay.tsx. So with fiat as the active unit and no rate available, scanning a BIP21 QR or opening Send with a preset amount puts the literal text "Disabled" into the editable AmountInput field.

This predates #4625: the Send constructor used the identical getUnformattedAmount(...).amount || satAmount expression before the extraction, and the param-update path used getAmountFromSats(), whose own sentinel is "N/A" (general.notAvailable, L268). Same bug class either way, so not a regression from #4625.

Impact

Low. getSatAmount() in fiat mode with no rate computes with rate = 0 and returns 0 for any input, so there is no NaN and no wrong-amount send. The symptom is sentinel text sitting in an editable input.

Why the obvious fix is not enough

Making the || fallback fire (raw sats string) would be wrong-unit: with the input in fiat mode, 12618 reads as 12,618 fiat units. The sentinel needs to be handled structurally instead:

1. getUnformattedAmount() already sets an error field in two of the three branches; add it to the no-currency branch too, and have callers key off error/unit rather than comparing display strings.
2. getRawAmountFromSats() should detect the failure and either return the sats-unit raw value while forcing the input's unit to sats, or return an empty string and leave the parsed satAmount intact.

Relation to existing issues

Same root-cause family as #2261 and #3831: amount formatters return localized sentinel strings instead of signaling failure structurally, and callers either render them ("N/A" everywhere in Amount.tsx) or, here, feed them into editable inputs. #3831 covers the display/UX side (and the FiatStore rate-clobbering behavior discussed there); this issue covers the editable-input side. A structural error signal from getUnformattedAmount() (point 1 above) would serve both.