fix: support WebP thumbnails inside ZIP archives

[yosofbadr]

Summary

Fixes #1364 by allowing WebP images inside ZIP/CBZ archives to be used as thumbnails.

This adds .webp to the archive thumbnail image extension checks, makes the ComicInfo cover extension check case-insensitive, and adds ZIP/CBZ WebP regression tests.

Tasks Completed

• Platforms Tested:
  • Windows x86
  • Windows ARM
  • macOS x86
  • macOS ARM - targeted local test/lint
  • Linux x86
  • Linux ARM
    • GitHub Actions also passed pytest on Linux and Windows for this PR.
• Tested For:
  • Basic functionality - targeted archive thumbnail regression tests
  • PyInstaller executable
    • Local: uv run --extra pytest pytest tests/qt/test_thumb_renderer.py -q
    • Local: uv run --extra ruff ruff check src/tagstudio/qt/previews/renderer.py tests/qt/test_thumb_renderer.py
    • Local: uv run --extra ruff ruff format --check src/tagstudio/qt/previews/renderer.py tests/qt/test_thumb_renderer.py
    • CI: Ruff, Pyright, REUSE, pytest Linux, pytest Windows, coverage all passed.

[yosofbadr]

Closing this because I missed #1373, which is already open for #1364 and covers the same area more completely.

Sorry for the duplicate review noise.