feat(dashboard): rework System Overview into a health status page

Reworks System Overview around one question: is the system healthy?

- A verdict banner, then three collapsible rows (parameters, control PC, services). Rows stay quiet when healthy and name the measurement when not.

- Percentages rounded to whole numbers (`20.995 %` → `21%`).

- Missing data reports `Unknown` instead of holding the last green value.

- Uses the same one-block + `h2` layout as the other pages, not nested cards.

Thresholds are configurable in a new `system-health.toml`.

Note: `ServiceUsageOverviewTable` now emits formatted strings (`"23%"`) instead of raw floats.`

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