`dev` does not build. This is fallout from #162: `vue-tsc -b` rejects
`section.value` with
error TS2339: Property 'value' does not exist on type 'Section'.
Property 'value' does not exist on type '{ name: "checksum"; letters: string; }'.
`Section` is a discriminated union and its "checksum" member carries
only `letters`. The predicate was written as `(s: any) => ...`, which is
not a type guard, so `find` returned the whole union and `.value` could
not resolve against every member.
Replaced with a real type predicate narrowing to the `amount` member.
My fault for how I verified #162: I ran `vue-tsc --noEmit`, but the
build script runs `vue-tsc -b`, which uses the project-reference config
and is stricter. The two disagree, so `--noEmit` alone is not evidence
that the build passes. #166 did run a full build, but it was branched
off #165 and so never contained this code.
Amountless behaviour is unchanged, re-verified after the narrowing:
amountless BOLT11 -> 0 (amount_msat sent)
live 777-sat -> 777000 (amount_msat omitted)
malformed -> null (defer to server)
`npm run build:wallet` now succeeds.
