[codex] add desktop source and target language controls

[MarsDoge]

What changed

• Added source and target language controls to the macOS menu-bar app.
• Added source and target dropdowns to the Linux GTK app.
• Added desktop version display actions backed by llm-translate --version.
• Updated macOS/Linux GUI docs and top-level README feature bullets.

Why

LLM providers can usually keep source language on auto-detect, but MyMemory-style translation APIs need an explicit source language for non-English source text. The desktop apps now make that distinction visible and configurable without editing env files.

User impact

• macOS users get Source Language, Target Language, and Show Version menu entries.
• Linux GTK users get Source, Target, and Version controls.
• Source defaults to Auto Detect; target still defaults from LLM_TRANSLATE_TARGET or Simplified Chinese.
• Choices only affect the current app process and do not write back to ~/.config/llm-translate/env.

Validation

• swift build in macos/LLMTranslateMac
• bash -n install.sh scripts/render-readme-diagrams.sh linux/install-desktop.sh linux/llm-translate-linux bin/llm-translate lib/openai_compat.sh lib/providers/*.sh
• shellcheck bin/llm-translate install.sh scripts/render-readme-diagrams.sh linux/install-desktop.sh linux/llm-translate-linux lib/openai_compat.sh lib/providers/*.sh macos/LLMTranslateMac/scripts/build-app.sh
• git diff --check
• printf '你好世界' | ./bin/llm-translate -p mymemory --source 'Simplified Chinese' --target English -> Hello world