fix(seed-demand): correct empty-state bed action label

[stipsitzm]

Motivation

• The seed-demand page empty state used inconsistent wording for the missing-beds action and displayed "Beete anlegen", which mixes wording patterns and is misleading because beds are managed on the Anbauflächen page.
• The goal is to use the established verb hinzufügen for user-created entities and ensure the CTA routes the user to Anbauflächen without opening the planting-plan form.

Description

• Updated the German translation for the seed-demand progressive empty-state beds action in frontend/src/i18n/locales/de/cultures.json from "Beete anlegen" to "Beet hinzufügen".
• Verified the seed-demand page action builds its CTA from the translation key and already routes to /app/fields-beds in frontend/src/pages/SeedDemand.tsx, so no route or component behavior change was required.
• Searched the codebase for other occurrences of the same seed-demand empty-state pattern and found no additional incorrect targets that required changes.
• Committed the change with the Conventional Commit message: fix(seed-demand): correct empty-state bed action label.

Testing

• No automated tests were executed (per request).

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Codex Task