Add GUI internationalization

[pepsing]

Summary

• Add gettext-backed GUI internationalization with Settings > General language switching.
• Add Chinese, German, French, and Spanish catalogs plus local extraction and compile tooling.
• Localize the main browser, settings, sidebar, playlist, photo, and podcast surfaces while preserving user and media titles as data.
• Improve default track table column fitting so generated widths fit the visible viewport before horizontal scrolling is needed.

Details

• Language selection is persisted in global settings and rebuilding the main window applies it.
• Catalogs are packaged via pyproject and documented in docs/i18n.md.
• iPod internal playlist categories are normalized to canonical labels before translation, so localized device databases still follow the chosen UI language.
• User-created playlist, album, photo album, and media titles are not translated.

Validation

• uv run pytest
• uv run ruff check
• uv run python scripts/update_translations.py --check
• git diff --check
• Manual launch on macOS with a connected iPod Video 5.5th Gen (Mojo); verified English playlist internal categories render in English.

Closes #113

[TheRealSavi]

thank you for all this work, i will be reviewing this PR and getting back to you or merging this week :)