Admin AI settings silently reset the assistant prompt, which cannot be edited in the UI

### Describe the bug

The AI assistant's prompt (`prompt_config`) can be stored and returned by the API, but there is no way to edit it, and saving AI settings silently overwrites whatever is stored with the built-in default.

`SaveSiteAI` substitutes the defaults whenever a request omits `prompt_config`:

https://github.com/apache/answer/blob/main/internal/service/siteinfo/siteinfo_service.go#L384-L389

```go
if req.PromptConfig == nil {
req.PromptConfig = &schema.AIPromptConfig{
ZhCN: constant.DefaultAIPromptConfigZhCN,
EnUS: constant.DefaultAIPromptConfigEnUS,
}
}
```

The admin form in `ui/src/pages/Admin/AiSettings/index.tsx` builds its payload from `enabled`, `chosen_provider` and `ai_providers` only, so it never sends `prompt_config`. Every save from that page therefore resets the prompt.

Combined, this means:

1. The prompt cannot be customised through the UI at all — there is no field for it.
2. A prompt set directly through the API survives only until somebody saves the AI settings page, then reverts silently with no warning and no indication in the UI that anything changed.

The backend side is fully implemented: `SiteAIReq` accepts `prompt_config`, `SaveSiteAI` persists it, `GetSiteAI` returns it, and `getPromptByLanguage` reads it and falls back to the constant when empty. Only the form is missing.

### Expected behavior

The prompt is editable in Admin → AI Settings, and saving other AI settings does not discard it.

### Steps to reproduce

1. Set a custom `en_us` prompt via `PUT /answer/admin/api/ai-config` with a `prompt_config` object.
2. Confirm it is stored (`GET /answer/admin/api/ai-config`).
3. In Admin → AI Settings, change nothing of substance and press Save.
4. Read the config again — the prompt is back to `DefaultAIPromptConfigEnUS`.

### Version

Reproduced on `main` (3b9f137) and on 2.0.2.

### Additional context

This surfaced while running the assistant against a vector-search plugin. `semantic_search` returns a `link` for every result, but the default prompt never asks the model to cite sources, so whether an answer links the thread it came from is left to the model. Being able to ask for citations in the prompt is the natural fix — which needs the prompt to be editable and to stay edited.

Happy to send a PR: the change is frontend-only, since the backend already supports the field.

Beginne in ui/src/pages/Admin/AiSettings/index.tsx und prüfe die Payload, die zum Speichern der AI-Einstellungen erstellt wird, und vergleiche sie anschließend mit dem vom Backend akzeptierten Feld prompt_config. Füge bearbeitbare Prompt-Felder hinzu und stelle sicher, dass der vorhandene Prompt beim Speichern anderer Einstellungen enthalten ist. Überprüfe, dass ein benutzerdefinierter Prompt nach dem Speichern des Formulars Admin → AI Settings unverändert bleibt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.