refactor: unify reusable add bed icon in hierarchy table and hint

[stipsitzm]

Motivation

• Reduce the visual dominance of the inline "+" add control and make it a secondary, subtle action across the hierarchy UI.
• Ensure the exact same visual is reused in both the table action and the informational hint to guarantee perfect visual consistency.
• Centralize styling and interactive behavior so the control can be toggled between interactive and decorative modes without duplicating styles.

Description

• Add a new shared component AddBedIcon that encapsulates all visual rules and interaction props (24x24 container, 14px icon, circular border, subtle green palette, 150ms transitions, interactive prop). (frontend/src/components/hierarchy/AddBedIcon.tsx)
• Replace the field-row add-bed action in the hierarchy table with the shared AddBedIcon used as an interactive control and wrapped in a Tooltip, preserving aria-label support and click handling. (frontend/src/components/hierarchy/HierarchyColumns.tsx)
• Replace the plain "+" in the missing-beds info box with the same AddBedIcon rendered as non-interactive/decorative (interactive={false}, aria-hidden) and aligned with text baseline. (frontend/src/pages/FieldsBedsPage.tsx)
• Split the German i18n hint string into before/after segments so the inline icon can be embedded without changing localized sentence flow. (frontend/src/i18n/locales/de/hierarchy.json)

Testing

• No automated tests were run for this change (per request).

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Codex Task