feat(ui): config-generator YAML preview + copy panel

[JSONbored]

Add the reviewable-output slice of the config generator (#1683): serialize the shared GeneratorFormState to .gittensory.yml text, render it read-only with a copy-to-clipboard button, so the output is 'explicit and reviewable before it is saved or copied' per the issue. Client-side serialization; single source of truth (same keys as the parser).

Deliverables

• Pure formStateToYaml(state) serializer in apps/gittensory-ui/src/lib/ emitting keys matching src/selfhost/private-config.ts (no parallel schema)
• Read-only YAML preview using the CodeBlock primitive (primitives.tsx:194) with copy button
• Handle empty/partial state (omit unset keys) so the preview is always valid YAML
• Vitest unit test of the serializer: full config, partial config (omitted keys), and empty state (every optional-key branch)

References

• apps/gittensory-ui/src/components/site/primitives.tsx:194
• apps/gittensory-ui/src/lib/maintainer-settings-preview.ts:146
• src/selfhost/private-config.ts

Part of #1683.

---

📸 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> |

---

---

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

[linear-code]

JSO-2311