fix: prioritize exact tag shorthand matches

[yosofbadr]

Summary

Fixes #1381 by ranking exact tag shorthand matches ahead of longer tag-name prefix matches in tag search results.

This changes Library.search_tags() so shorthand text is considered when sorting match quality, and adds a regression test for the reported blue / black case where query b should rank the tag with shorthand b first.

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 search ranking regression test
  • PyInstaller executable
    • Local: .venv/bin/python -m pytest tests/test_library.py -q
    • Local: .venv/bin/ruff check src/tagstudio/core/library/alchemy/library.py tests/test_library.py
    • CI: Ruff, Pyright, REUSE, pytest Linux, pytest Windows, coverage all passed.