Add configurable AI models folder

[muaz978]

Summary

Fixes #14024.

Adds an optional AI models folder setting so large downloaded model files can be stored on an external SSD without moving Subtitle Edit's settings, logs, dictionaries, themes, or executable folders.

What is covered

• Adds Options > General > AI models folder with a folder picker.
• Preserves all historical locations when the setting is empty.
• Redirects managed Whisper, CTranslate2, Whisper.cpp, Purfview, CrispASR, TTS, Qwen3 ASR, llama.cpp, PaddleOCR, CrispEmbed, Tesseract, and audio.cpp model paths.
• Uses XDG_CACHE_HOME for OpenAI Whisper and HF_HOME for CTranslate2 when a custom root is selected.
• Keeps engine binaries in the normal application-data folder.
• Adds focused compatibility tests and detailed implementation documentation.

Migration behavior

Existing models are not moved automatically. The setting changes future model resolution and downloads, while the documentation explains the manual copy layout. This avoids destructive or interrupted multi-gigabyte moves.

Verification

AVALONIA_TELEMETRY_OPTOUT=1 dotnet test tests/UI/UITests.csproj --no-restore --filter FullyQualifiedName~DataFolderLocationTests

Result: 6 passed, 0 failed.

Also verified with git diff --check.