fix(l10n): add German for untranslated strings

[luflow]

Summary

Fills the German translation gap for 74 source strings that had no entry in the German l10n files yet (mostly the mobile self-check-in/NFC/QR block in src/App.vue, subscription/feedback dialogs, and activity strings).

• Adds German (informal "du", existing wording conventions) to l10n/de.json, l10n/de.js, l10n/de_DE.json, l10n/de_DE.js
• de/de_DE differ by 4 keys that already existed in de_DE (74 vs 70 additions)
• Placeholders ({time}, {state}, %n, markdown links) and the non-breaking space before … kept byte-for-byte
• Purely additive diff, no reordering; find_untranslated.py now reports 0 missing, find_stale.py reports no orphans, npm run build passes

Note: this is the intended stopgap from the l10n-autotranslate skill — entries will be re-matched/overwritten on the next Transifex sync.

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