Wire API-Route prompt into translate settings

[Blackspirits]

Summary

Upstream PR SubtitleEdit#14836 adds ApiRoutePrompt to both runtime and persisted auto-translate settings, but does not register ApiRouteTranslate in TranslateSettingsViewModel.

As a result, the per-engine Translate settings dialog treats API-Route as an engine without a configurable prompt:

• the prompt editor is hidden because GetPrompt returns null;
• Reset prompt cannot restore the API-Route default;
• SaveValues never writes an edited API-Route prompt;
• the normal {0} / {1} source/target placeholder validation is skipped.

This change mirrors the existing OpenRouter/Groq/etc. wiring:

• include API-Route in prompt validation;
• persist ApiRoutePrompt to both Se.Settings and Configuration.Settings.Tools;
• return the API-Route prompt from GetPrompt.

Validation

• based exactly on upstream PR feat(translate): add API-Route auto-translator SubtitleEdit/subtitleedit#14836 HEAD d47d366133e62a876b72c598289a537b41170235
• one UI view-model file only
• no API transport/model behavior changed
• full solution restore passed
• full solution build passed
• full solution test suite passed on the first run
• UI suite: 5,149 passed, 9 skipped, 0 failed (5,158 total)
• retry step was not used
• validation run: #34780482268

Note: API-Route's current documentation says supported models change over time and the Pricing page is the source of truth. SubtitleEdit#14836's static model suggestions are therefore maintenance debt, but Model is a free-text field, so that is not part of this functional prompt fix.

AI assistance: ChatGPT was used to adversarially review the new API-Route auto-translator's settings/persistence integration.

[Blackspirits]

Independent adversarial re-check: upstream SubtitleEdit#14836 adds ApiRoutePrompt to runtime/persisted settings but omits ApiRouteTranslate from TranslateSettingsViewModel's prompt contract. Without this patch the prompt editor is hidden, reset/save do not handle ApiRoutePrompt, and {0}/{1} placeholder validation is skipped. The final diff mirrors existing OpenRouter/Groq wiring in exactly those three places and changes no API transport/model behavior. Full CI #34780482268 passed restore, build and the complete suite on the first run; UITests reported 5,149 passed, 9 skipped, 0 failed. Retry was not used. No blocker identified in the fix. Keep draft; no merge performed.