Comparison overlay: same metric over the previous window

The standard "is this normal?" affordance: a toggle that overlays a ghost/dashed line of the same metric from the previous window (yesterday / last week, matched to the preset) time-shifted onto the current chart. Requires one extra query per panel with the shifted range (the query layer already parameterizes start/end) and a muted series style.

## Acceptance criteria
- Toolbar toggle "Compare: previous period" (URL-encoded, e.g. `compare=1`, in `statusSearch.ts`; default off).
- When on, each line chart overlays the same metric from the immediately preceding window of equal length, time-shifted to align, rendered dashed/muted and legend-labelled "(prev)"; tooltips label both series sets.
- No extra fetches while the toggle is off; presets only (disabled for a custom historical range if #1456 lands first).

## Implementation notes
- One extra `useAnalyticsRecords` call per panel with the shifted window, gated on the toggle; shift each point's x by the window duration before rendering; muted style via `opacity`/`strokeDasharray` on the existing `Series` fields.

Filed from a Status tab code review.

Comment generated by kAIle (Claude Fable 5)