net/vnstat: dashboard widget

[joeroback]

Important notices
Before you submit a pull request, we ask you kindly to acknowledge the following:

• I have read the contributing guide lines at https://github.com/opnsense/plugins/blob/master/CONTRIBUTING.md
• I opened an issue first for non-trivial changes and linked it below.
• AI tools were used to create at least part of the code submitted herewith.

If AI was used, please disclose:

• Model used: claude code (Opus 4.6 (1M context))
• Extent of AI involvement: claude wrote the basic layout and helped with some of the date/time calculations

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Related issue
If this pull request relates to an issue, link it here:

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Describe the problem
Vnstat dashboard widget

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Describe the proposed solution
Adds vnstat dashboard widget. Allows you to select interface, and period (hourly, daily, monthly, yearly)
It remembers the last selected interface/period
Preferences to select the update tick interval, defaulting to 5 minutes

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