chore: resolve remaining repository checker findings

[DutchmanNL]

Second-wave cleanup for the open repository-checker issue #365. All errors from the first wave are already resolved on main; this PR clears the remaining actionable warnings/suggestions.

Fixed

• W5604 — admin/i18n/<lang>/translations.json for de, es, fr, it, nl, pl, pt, ru, uk, zh-cn were each missing the English key Seconds between data polling (the intervall jsonConfig label). Added a proper translation to all ten files.
• S5005 — the data-polling timer in main.js used plain setTimeout()/clearTimeout(). Converted to this.setTimeout() / this.clearTimeout() inside the adapter class so the timer is auto-cleared on unload.
• S5032 — removed lib/tools.js, which was dead code (not required anywhere in the repo); also dropped its stale mention from .github/copilot-instructions.md.

Leftovers (deliberately not changed)

• W3009 (concurrency block in test-and-release.yml) — the checker wants the literal upstream form, but our form deliberately protects release/tag runs from cancellation. Accepted, documented deviation.
• W3052 / S3056 (workflow logs no longer retrievable for old runs) — nothing to fix in the repo; these age out and will clear on the next workflow runs.
• S5601 (migrate admin/i18n from {lang}/translations.json to short {lang}.json) — optional format migration, left for a dedicated change rather than bundling it with a fix PR.

Local verification

• npm install --no-audit --no-fund — OK
• npm run lint — 0 errors (3 pre-existing JSDoc @param description warnings, untouched)
• npm run test:package — 57 passing
• npm run test:integration — 1 passing
• npm run test:integration-demo (the command CI runs) — 2 passing

Note: main.test.js contains a leftover adapter-template dummy test that fails on the should() syntax; it is pre-existing, unrelated to this change, and is not executed by CI or by any of the checker-relevant scripts.

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