i18n(l10n): complete en/nl runtime translation coverage

[rubenvdlinde]

Summary

• Completes runtime l10n coverage for l10n/en.json + l10n/nl.json. Both files now pass npm run test:l10n (0 missing keys, full en/nl parity).
• The static regex-based gap scanner initially reported false positives (truncated keys from strings containing embedded literal quotes) and missed real gaps hidden behind dynamic t('procest', someVariable) calls that resolve to values from local lookup-table constants (STATUS_LABELS, STEP_LABELS, GENERIC_ROLE_LABELS, VOORSTEL_TYPE_LABELS, VOORSTEL_STATUS_LABELS, ACTION_LABELS, hearing-status literals, and the Bezwaar/Beroep landing-page card labels/descriptions). Those constants were traced by hand and their literal values added.
• 31 new key/value pairs added to both files (English source keys are identity per the project's --write convention; Dutch values authored using correct NL-gov case/objection/appeal terminology, reusing existing wording where the app already had matching concepts, e.g. Geparafeerd, Overgeslagen, Besluitvormer, Coördinator).
• No existing key was changed or removed. No .js, src/, register, or other-locale files touched.

Test plan

• npm run test:l10n → OK (2143 used keys, 2930 keys in en.json, en/nl parity match)
• python3 -c "import json; json.load(open('l10n/en.json')); json.load(open('l10n/nl.json'))" → valid JSON
• git diff reviewed — purely additive on l10n/en.json and l10n/nl.json
• appinfo/info.xml version bumped 0.3.6 → 0.3.7 (patch, no register/schema change)