[Lukasjoosten]

Refs #2864

Adds Dutch (nl) to the Apple apps (macOS, iOS, watchOS), as the counterpart of the Android Dutch localization in #2865. It follows the shape of the Italian localization (#2454).

What changes

• Catalogs: every translatable key gains an nl unit in Strand (4126), StrandDesign (57), NOOPWatch (69) and NOOPWatchComplications (30). Keys with plural variations in any locale get Dutch one/other forms (79 keys).
• Reuse: about 1,400 strings reuse the reviewed Android Dutch wherever the English source is identical. Android placeholders are mapped to the Apple shapes (%1$s → %@, %1$d → %lld, positional only where Dutch reorders arguments).
• AppLanguage: gains case dutch = "nl" after italian, with the autonym "Nederlands", and a test mirroring the Italian one.
• Baselines: i18n_extra_locale_baseline.txt gets nl 0 for all four catalogs, so Dutch fails CI on any new English-only string. i18n_echo_baseline.txt gets Strand … nl 27 and NOOPWatchComplications … nl 2.

Verification

• Every translatable key has a translated nl unit.
• For every form, the placeholder multiset matches the key and the %% counts match.
• Newline counts match the English. The exception is three tempo strings ("Regular tempo…", "Mid session tempo…", "Punching Through…"): only their English value carries a leading \n\n that no other locale has, so the Dutch follows the key and the other locales.
• All plural keys have one and other, and all four catalogs parse as JSON.
• No re-serialisation: the catalogs were appended rather than re-serialised, and removing the inserted text reproduces the original bytes. 411 entries in Strand sit on a single line, so they are necessarily edited in place (insertion only). Everything else is pure line additions.
• Echo counts: recomputed with the audit's own predicate. Each entry is a brand, a metric abbreviation, a proper name kept as on Android (Presence, Wim Hof), or wording that is already correct Dutch ("%@ in bed", "Per extra %@").
• i18n_audit.py --ci upstream/main: clean for nl. Locally on Windows the only FAIL/STALE pairs are the known \ vs / path artifact for ru and zh-Hant.
• unittest discover -s Tools: 338 tests, with the same 4 test_check_source_references failures as a clean upstream/main copy on the same Windows machine.
• Swift not compiled locally: the changes were made on Windows without Xcode, so AppLanguage.swift and AppLanguageTests.swift are validated only by app-build.yml on this PR.

Translation

The translations were machine-assisted against a fixed glossary and reviewed by a native Dutch speaker (the PR author). The glossary is the one approved for the Android half: informal "je", sentence case, NOOP's own Charge/Effort/Rest → Lading/Inspanning/Rust, kept distinct from the WHOOP terms Recovery/Strain → Herstel/Belasting.

Noticed while translating (not changed here)

• Two Fitness Age accessibility labels interpolate an English-only plural "s" into the localized string (yr%@, year%@), so non-English locales get a stray suffix.
• MarkerEditorView parses values with Double(_:), which rejects a decimal comma, so the Dutch hint keeps "bijv. 3.1". This affects every comma-decimal locale.

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