feat(deployments): mark deployment start and end on activity sparklines

[Chouffe]

Summary

• Deployments tab: each bars/heatmap sparkline gets two faint 1px vertical marks at the deployment's first and last bucket edges, so "camera wasn't running" reads differently from "camera ran but saw nothing". Faint slate at rest, blue on row hover or selection. The marks sit behind the chart (negative z-index in an isolated wrapper) so the data always paints on top. Section headers for co-located deployments show the union of their children's ranges in muted slate. Line mode is deliberately left without marks because they fight the smoothed curve.
• The cursor count pill on the selected row now reads "not deployed" instead of "0 obs" when the cursor is over the selected row itself, the hovered bucket is empty, and it lies entirely outside that deployment's window. Buckets with observations always show their count, and hovering any other row keeps the plain count (the pill always describes the selected deployment).
• Fixes a pre-existing misalignment surfaced by this work: indented child rows under a location header shifted their sparkline 28px right of the header's date axis and the hover ruler, so the crosshair mapped to the wrong bucket on grouped deployments. The indent is now absorbed by the name column.
• New pure helper coverage.js snaps dates outward to bucket edges (half-open buckets, open-ended on a missing date, null on unparseable or out-of-grid input). Renderer-only; no IPC or query changes, the activity payload already carried deploymentStart / deploymentEnd.

Test plan

• node --test test/renderer/*.test.js: 257 pass (16 new: bucket snapping, boundary dates, open-ended ranges, unparseable dates, group union incl. order-independence with bad dates)
• Driven over CDP on the 411-deployment / 154-location study at 2x DPR: every mark within 1px of a sparkline cell edge in bars and heatmap; 0 marks rendered in line mode; z-index and isolation verified from computed styles; child-row sparklines start at the same x as their header's
• Pill on a selected child row: "259 obs" hovering its deployed bucket, "not deployed" hovering outside; "0 obs" when hovering another row's busy cell
• Eyeball dark mode on a study with a long gap between deployments at the same location

[netlify]

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[Chouffe]

would love your review @ioRekz