chore(build): type-check what ships (tsconfig.build.json) and move to Next 16.3.1 (#1750)

* chore(build): type-check what ships (tsconfig.build.json) and move to Next 16.3.1

Groundwork for the 2026 shape of the build, companion to #1749.

- tsconfig.build.json extends tsconfig.json and excludes tests/, __tests__,
  __mocks__ and *.test.ts(x); next.config.ts selects it via
  typescript.tsconfigPath. tsconfig.json is untouched and stays the
  editor/ESLint view of the whole repo.
- next 16.2.12 -> 16.3.1 (+ eslint-config-next). 16.3 runs the project-local
  tsc CLI by default, which is what lets typescript@^7 (native) slot in once
  typescript-eslint supports the TS 7.1 API, and turns on Turbopack's on-disk
  cache for next build.

The split has to land with the bump: the 16.3 CLI checker checks the
complete project it is given, while the old API checker silently dropped
diagnostics from test files. A full tsc --noEmit of main reports 493 type
errors, all in tests (mostly route handlers called without the ctx
argument); vitest never type-checks, so nothing caught them. Excluding tests
from the build keeps that debt where it was instead of turning it into a
red deploy; a tests type-check job is the follow-up. Measured: tests are
~10% of the check's memory, so this is correctness, not the memory fix.

Lockfile regenerated with npm 10 (CI pins node 20); the only structural
change is npm 10 de-nesting next-intl/node_modules/@swc/helpers.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(build): only write standalone output off-Vercel; Vercel's 16.3 adapter owns tracing

The #1750 preview failed after a green compile and type-check with
ENOENT .next/next-server.js.nft.json right after "Running onBuildComplete
from Vercel": writeStandaloneDirectory copies from that trace file, and on
Vercel (adapterPath set) Next 16.3 no longer produces it; the adapter traces
and packages functions itself. Vercel never reads .next/standalone; the
Docker image does. Keep standalone for every non-Vercel build.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

DECISIONS.md

@@ -1118,3 +1118,4 @@ One line per decision: `[YYYY-MM-DD] <decision>: <why>`. Appended by agents and
 [2026-08-20] The matcher now runs automatically once per window+account when there is unmatched work, instead of waiting for a button many users never found. It is a dry run: nothing is written, and Tillampa still requires an explicit click. ?autorun=1 keeps a distinct meaning (run even on a clean window) so the transactions-inbox deep link still produces a result rather than silence.
 [2026-08-20] Unmatched bank rows that no voucher on the account could settle (direction-compatible and equal to the ore) get "Bokfor" linking to /transactions?highlight=<id> instead of a match picker. They are unbooked affarshandelser, not reconciliation work, and the picker held nothing for them. The rule is deliberately strict: a false negative offers booking on a pairable row (a legitimate outcome), a false positive sends the user into an empty picker.
 [2026-08-20] Vercel build heap is raised through vercel.json `buildCommand` (`NODE_OPTIONS=--max-old-space-size=6144 npm run build`), not a project env var and not `build.env`: a project-level NODE_OPTIONS also reaches function runtime (V8 sizes the heap against a limit the function does not have), and `build.env` is marked deprecated in the vercel.json schema; `buildCommand` scopes the flag to the build exactly like core-build.yml's 8192 does for CI. 6144 fits the standard 4-core/8 GB build machine next to the main next process; the type-check needs ~4.5 GB and was hanging at V8's ~4 GB default ceiling (4 production timeouts 2026-08-14..20).
+[2026-08-20] The production build type-checks tsconfig.build.json (tsconfig.json minus tests and mocks) via typescript.tsconfigPath; tsconfig.json stays the editor/ESLint view of the whole repo. Why: Next 16.3 runs the project-local tsc CLI by default (prerequisite for TypeScript 7's native checker, which has no JS API) and that checker checks the complete project it is given, whereas the old API checker silently dropped diagnostics from __tests__/*.test.* files. ~490 real type errors live in test files today (mostly route handlers called without the ctx argument); vitest never type-checks them, so nothing caught them. Excluding tests from the build keeps that debt where it was (invisible) instead of turning it into a red deploy; a separate tsc job for tests is the follow-up that makes it visible. Measured: tests are ~10% of the check's memory, so this is correctness, not the memory fix (that is the vercel.json heap bump).