Auto-translate: add reset-to-default button for the prompt

[niksedk]

Both prompt editors in auto-translate let you edit the prompt but gave no way back to the built-in text once it was changed. This adds a small ↺ icon button next to each Prompt text label:

• Auto-translate → settings (cog) — restores the built-in prompt of the currently selected engine (ChatGPT, Anthropic, Gemini, Ollama, llama.cpp, …).
• Auto-translate → llama.cpp → Advanced… — fills the box with LlamaCppAdvancedProtocol.DefaultPrompt (that box is normally empty, with the default only shown as placeholder text).

Both buttons use mdi-restore and carry the accessible name / hint "Reset prompt to default" (new LanguageTranslate.ResetPromptToDefault string).

The 13-engine if/else chain in TranslateSettingsViewModel.LoadValues is collapsed into one GetPrompt(settings, engineType) helper, used for both the saved prompt and the built-in default — the reset command and the loader read the same mapping, so they cannot drift apart.

Testing

New headless tests in tests/UI/Features/TranslateSettingsResetPromptTests.cs:

• the button reaches the logical tree and is visible for an engine with a prompt,
• it is hidden for an engine without one (the whole prompt row hides),
• the command replaces a customized prompt with the built-in default.

All 10 tests in the related filter pass.

🤖 Generated with Claude Code