Add German translation with key and placeholder checks

[nawaaaaaAaar]

Summary

Closes #97. Add the requested German translation for Knuspr.de and Gurkerl.at users, covering setup, reauthentication, options, selectors, entity names and states. Add offline checks for the complete string-key tree and exact interpolation placeholders.

Verification

• python -m unittest discover -s tests -p test_translations.py -v: two tests passed on Python 3.12 and 3.13, with per-string subtests.
• UTF-8 JSON parses successfully; every English leaf key exists, values are nonempty strings and {email} placeholders remain unchanged.
• No production Python or API behavior changed.

Limits

AI-assisted translation prepared through Perplexity Computer. Native-speaker and live Home Assistant UI review remain valuable; they have not been performed. The Home Assistant integration suite, hassfest and HACS checks are not claimed as local passes. No customer account or credentials were used.

[dvejsada]

@claude review

[claude]

Claude finished @dvejsada's task in 22s —— View job

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Review of PR #104: German translation

• Read de.json and compare it with en.json
• Read tests/test_translations.py
• Run the tests. I couldn't: the shell here blocked python -m unittest pending approval.

Verdict: looks good to merge, with a few optional nits. I found no blocking issues.

Translation (de.json)

• Structure: reading both files side by side, de.json has the same key tree as en.json. That covers config steps, errors, aborts, options, selectors, binary sensors, sensors, todo and calendar.
• Placeholders: {email} is intact in reauth_confirm and reconfigure.
• Tone: it uses the informal "du" consistently, which suits Home Assistant.
• Wording: the terms read naturally. Examples are "Mehrwegtaschen", "Lieferzeitfenster", "Nicht mehr angebotene Produkte" and "Stk." for pcs.
• Nit, formatting: de.json writes short objects inline, such as "state": {"on": "Ja", "off": "Nein"} and {"name": "..."}. en.json and (I assume) cs.json use expanded multi-line objects. This is valid JSON, but expanding it would match the other files and keep future diffs consistent.
• Nit, config.step.user.title: "Rohlík-Konto" is right for CZ. German users with Knuspr.de or Gurkerl.at may find "Shop-Konto" less confusing. This mirrors the English "Rohlík account", so it's optional.
• Nit, bags_amount: the name "Anzahl der Taschen" is fine. The English name is "Bags Amount", so "Taschen" alone would also work.
• A native-speaker pass is still worth doing, as the PR description says.

Tests (tests/test_translations.py)

• The two tests are sound. They check that the key sets match, that values are non-empty strings, and that placeholders are identical.
• Nit, failure messages: assertEqual(english.keys(), german.keys()) reports only that the key views differ. Asserting on the set difference would name the missing or extra keys:

  self.assertEqual(set(english) - set(german), set(), "missing in de")
  self.assertEqual(set(german) - set(english), set(), "extra in de")

• Nit, naming: TestGermanTranslation only covers de, but the logic is generic. Parametrising over every translations/*.json (including cs.json) with subTest would cover all locales for little extra code. The file-loading code is also duplicated between the two tests and could move into setUpClass.
• CI: I didn't check whether any workflow runs tests/. If none does, this check only protects contributors who run it locally.

[nawaaaaaAaar]

Addressed the formatting and failure-message nits: de.json now uses the same expanded two-space style as en.json, and parity failures name missing/extra keys. Both offline tests were rerun successfully on Python 3.12 and 3.13. Translation wording is unchanged; native-speaker and live UI review remain explicitly unperformed. The existing Tests workflow does include tests/ once its run is approved.