chore(js-client): upgrade to TypeScript 6

[claude]

Requested by Łukasz Komoszyński · Slack thread

Upgrades the js-client/ package to TypeScript 6.0.3, done as a real tsconfig migration rather than by silencing the TS 6 deprecation errors.

Stacked on #92 (chore/js-client-npm-dependencies-bump). This PR's base is that branch, so its diff shows only the TypeScript-6-specific changes. Review/merge #92 first; GitHub will retarget this PR to main once #92 lands.

Why this is a separate PR

#92 briefly carried the typescript 5.9.3 → 6.0.3 bump using "ignoreDeprecations": "6.0" in tsconfig.base.json. That only mutes the errors and defers the real work. #92 has been reverted back to TypeScript 5.9.3 (dropping ignoreDeprecations), and the upgrade lives here instead, migrated properly.

What changed

• typescript ^5.9.3 → ^6.0.3. typescript-eslint@8.64.0 already supports it (peer typescript <6.1.0).
• Dropped baseUrl. TS 6 turns the deprecated baseUrl into a hard build error (TS5101). It was only present to anchor the @leancodepl/pipe path mapping; TypeScript has resolved paths relative to the config file without baseUrl since 4.1, so the mapping is now a relative specifier:

  // tsconfig.base.json
  "paths": { "@leancodepl/pipe": ["./packages/pipe/src/index.ts"] }

  No source imports relied on baseUrl-relative bare specifiers, so nothing else needed rewriting.
• moduleResolution "node" → "bundler". TS 6 also hard-errors on the deprecated "node" (aka node10) resolver. The pipe library is bundled through @nx/rollup + swc, so "bundler" is the correct fit — and it matches js_corelibrary's org-preferred tsconfig.base.json. module stays ESNext (compatible with bundler).
• No ignoreDeprecations. The escape hatch is intentionally absent; tsc reports zero TS5101 deprecation errors under the migrated config.

Test status

Verified on Node 24.18 (.nvmrc): nx lint pipe and nx build pipe are green, and tsc -p packages/pipe/tsconfig.lib.json --noEmit reports no errors. Tests pass except the single live-backend integration test (packages/pipe/__tests__/pipe.spec.ts), which fails only because the sandbox blocks api.exampleapp.test.lncd.pl by egress policy (403) — the same environmental caveat as #92, not a code failure. It is expected to pass in CI.

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