Branding page: import a v1 theme.json and export it as a v2 appearance document

## Background

Since FR-3605 (PR #9361) the appearance loader reads the **v2** document only (`schemaVersion: 2`, `{ theme, branding `}). A legacy v1 `theme.json` (antd-shaped `light`/`dark` token blocks) fails schema validation and is not applied — the app falls back to the Astryx neutral theme with a console error. Operators with an existing v1 document have no in-product path to migrate it.

## Goal

On the Branding page, accept an existing v1 `theme.json`, convert it to the v2 structure, and let the operator export (download) the converted document.

## Scope

- Import: file picker / paste for a v1 document; detect the v1 shape (`light.token`, `dark.token`, `components.Layout.headerBg`, logo fields).
- Convert to v2: `colorPrimary/colorLink/colorInfo/colorError/colorSuccess` → `theme.families.default.seeds.{accent,link,info,error,success`} as `[light, dark]` tuples (dark values as declared — no darkAlgorithm re-mapping), `fontFamily` → `theme.fontFamily`, `Layout.headerBg` → `families.default.headerBg`, logo/company fields → `branding`; drop v1 keys that have no consumer (see the FR-3605 v1 → v2 field map).
- Validate the result against `resources/theme.schema.json`; surface validation errors instead of silently defaulting.
- Preview in the Branding editor and export the v2 JSON.

## Acceptance

- A v1 document produces a v2 JSON that the loader accepts without console errors.
- Validation failures are shown to the operator with the offending path.
- Dark values in the export equal the declared v1 dark seeds (no transform).

## References

- FR-3605 / PR #9361 — v2 appearance document, theme-shim retirement, migration field map in the PR body.

JIRA Issue: FR-3847

Start with the Branding page/editor and the migration field map in FR-3605 / PR #9361, then inspect resources/theme.schema.json for the v2 shape and validation paths. Implement the v1 import, preview, validation-error reporting, and export flow; done means the loader accepts the exported v2 JSON, errors identify offending paths, and dark seeds remain unchanged.

Written by the indexing model from the issue text.