feat(ui): findings-by-category/severity breakdown card

[JSONbored]

Add one analytics card that breaks down AI-review findings by category and severity from the stats feed. Single grouped-bar / stacked-count card; read-only over an existing payload.

Deliverables

• New FindingsBreakdownCard consuming findings-by-category/severity from src/review/stats.ts (StatsPayload)
• Grouped rows per category with severity counts; color via existing token classes only (no hardcoded hex)
• EmptyState (state-views.tsx:86) when no findings in window; window-days label as a StatusPill
• Wire into apps/gittensory-ui/src/routes/app.analytics.tsx
• Vitest test: multi-category populated, single-category, empty (both branches of the count>0 guards)

References

• src/review/stats.ts:165
• apps/gittensory-ui/src/components/site/usage-analytics-panels.tsx:235
• apps/gittensory-ui/src/routes/app.analytics.tsx

Part of #1967.

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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-2296