fix(gantt-chart): show backend validation errors and rollback failed updates

[stipsitzm]

Motivation

• The Gantt calendar previously displayed only a generic "Fehler beim Aktualisieren des Anbauplans" message for backend 400 validation responses and left optimistic UI changes visible despite the backend rejecting the update.
• Users need to see the same field-level validation messages (e.g. area_usage_sqm) as in the Planting Plans view and the calendar must remain consistent with the backend after a failed update.

Description

• Reused the shared API helper by importing extractApiErrorMessage and used it in handleTaskUpdate to show localized field-level validation messages instead of a generic error.
• Added a refreshPlantingPlans function and call it in the update failure path to reload planting plans from the backend and revert any optimistic changes.
• Force a Gantt remount by introducing ganttRenderKey when an update fails to clear any stale internal drag/resize state in the chart component.
• Kept successful update behavior unchanged (clear error and refresh weekly yield), and added tests simulating both failed (400 with area_usage_sqm) and successful update flows.

Testing

• Ran npm test -- --run src/__tests__/GanttChart.test.tsx and all tests passed (9 tests, 9 passed).
• Tests verify that a 400 response containing area_usage_sqm is displayed as the concrete backend message, that the planting plans are reloaded after a failed update (rollback), and that successful updates continue to work as before.

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Codex Task

[stipsitzm]

@copilot fix failing test or the code