refactor(price): answer the price fetch on one typed outcome channel

[zancas]

What this changes

getZecPrice returned one number that carried both the price and the failure.
Zero, minus one and minus two each named a different outcome. A prose string
travelled beside the number.

callFfi already gives this function a discriminated FfiResult. Its code
is a ZingolibError variant name. The sentinels discarded that typing.

The function now returns a ZecPriceOutcome. One arm is zecPrice and holds
the quote. The other arm is ErrorKeyed and holds a catalog key with the
diagnostic detail. ADR 0002 and ADR 0003 ask this of every module boundary.
The file's own header already claimed it.

What the sentinels were hiding

Three defects go away with them.

• NaN is falsy. The guard above the isNaN arm therefore always won, and
  that arm was unreachable. A malformed number reported as "no price yet".
• The payload's error field is dead. zec_price returns only
  current_price. It signals every failure by rejecting.
• The store retried every non-positive answer. A deliberate mixnet-off refusal
  under ADR 0011 therefore cost two failing calls, not one clear answer.

A quote must now be a positive finite number. An absent, null or non-positive
value fails as a malformed payload. It cannot pass as a price.

Behaviour change

A failed refresh now always leaves the last good price on screen. Two of the
three old failure arms did that. The third overwrote the price with zero. The
same fetch failing two ways therefore produced opposite displays.

Scope

This is a mobile backend change. The snackbar copy is untouched, and the two
translation keys keep their present wording. Those two strings read almost
identically, so a reader cannot tell the two failures apart. That is a
user-facing concern and belongs in its own change.

Verification

• npm run typecheck passes.
• npx eslint reports nothing on the four changed files.
• npx prettier --check passes.
• The walletUtils unit suite passes with 24 tests. Its price cases now
  assert the error key rather than a sentinel, and they cover a null price and
  a non-positive price, which the old shape could not express.
• The PriceFetcher snapshot passes unchanged.

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