feat(ui): slop + duplicate trend-over-time card (maintainer dashboard)

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Add a card to the maintainer quality dashboard charting slop-flag and duplicate-flag rates over time from existing snapshots (weekly-value-report / collision clusters per #539). Single dual-series trend card; bands not raw scores.

Deliverables

• New SlopDuplicateTrendCard in apps/gittensory-ui/src/components/site/app-panels/
• Two TrendChart series (slop rate, duplicate rate) with a shared legend; band labels not raw credibility numbers
• generatedAt + stale flag surfaced (consistent with feat(ui): maintainer repo/org quality dashboard #539 snapshot-read rule)
• Wire into MaintainerPanel (maintainer-panel.tsx)
• Vitest test: both series present, one-series-empty, and no-data branches

References

• apps/gittensory-ui/src/components/site/app-panels/maintainer-panel.tsx
• apps/gittensory-ui/src/components/site/trend-chart.tsx:8
• src/services/weekly-value-report.ts

Part of #539.

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📸 Required: before/after screenshots (this is a UI change)

Non-negotiable. PRs that change UI must include a before/after screenshot table. PRs without it are automatically closed — no exceptions.

Rules:

• Show every page/feature you changed — before and after, one row each.
• Use the markdown table below. Screenshots go inside the table only — no large full-size images pasted elsewhere in the PR.
• Each cell is a clickable thumbnail (a small image linking to the full-size image) with a one-line caption underneath.
• Upload screenshots to the PR by drag-and-drop (GitHub hosts them on its CDN). Do NOT commit image files to the repo.

Copy this and fill it in (replace FULL_IMAGE_URL with the URL GitHub gives you after you drop the image in):

| Page / Feature | Before | After |
|---|---|---|
| `/your-route` | [<img src="FULL_IMAGE_URL" width="260">](FULL_IMAGE_URL)<br><sub>before: short caption</sub> | [<img src="FULL_IMAGE_URL" width="260">](FULL_IMAGE_URL)<br><sub>after: short caption</sub> |

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size: S · held for later release (gittensor:feature once activated)

[linear-code]

JSO-2303