Backrest widget fails with "Invalid plans data" when only repoSummaries are returned

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Discussion Link

#5837

Additional context

Hi, 
I’m using the new Backrest widget introduced in v1.5.0. My Backrest setup does not use any backup plans, only repositories. When the widget calls `/v1.Backrest/GetSummaryDashboard`, the API response contains only repoSummaries but no planSummaries.

Example response from my Backrest server:

{
  "repoSummaries": [
    { "id": "repositoryOne", "recentBackups": {} },
    { "id": "repositoryTwo", "recentBackups": {} }
  ],
  "configPath": "/config/config.json",
  "dataPath": "/data"
}

Homepage then shows the following error:

API Error: Invalid plans data
URL: http://<ip-backrest>:9898/v1.Backrest/GetSummaryDashboard

Expected behavior: The widget should handle the case where planSummaries is missing or empty, and still display the repository summaries (repoSummaries).

Actual behavior: The widget fails with Invalid plans data because it expects planSummaries to always be present.

Steps to reproduce:
- Configure Homepage with the Backrest widget.
- Use a Backrest instance that has repositories but no plans.
- Open the dashboard → error appears.

Environment:
- Homepage version: 1.5.0
- Backrest version: 1.9.2
- Deployment: Docker

Suggestion: Allow the widget to gracefully handle responses that only contain repoSummaries (e.g. treat missing planSummaries as an empty array).