feat(naming): add {FirstAuthor} template token

[viniciuspx]

Closes #930.

What

New {FirstAuthor} naming-template token. It renders only the first author when metadata lists several ("Author1, Author2, Author3"), so multi-author books can be filed alongside the rest of that author's work instead of getting their own "Author1, Author2, ..." folder.

{Author}       -> Terry Pratchett, Neil Gaiman
{FirstAuthor}  -> Terry Pratchett

How

• Added to KNOWN_TOKENS in shelfmark/core/naming.py, positioned before author so {FirstAuthor} isn't parsed as literal First + {Author}.
• Derived inside parse_naming_template from the existing Author value (split on , / ;), so every caller — folder transfer, rename, the settings preview — picks it up with no extra wiring. An explicit FirstAuthor key in the metadata still wins if one is ever passed.
• {Author} behaviour is unchanged.
• Frontend namingTemplatePreview.ts token list + KNOWN_TOKENS kept in lockstep (there's a test enforcing that), with a matching firstAuthor helper.
• Settings field descriptions + docs/environment-variables.md list the new token.

Known limitation

A lone author written Last, First is split on the comma too and renders as Last — the source metadata doesn't mark which form it is. Called out in the token help text and covered by a test. {Author} remains available for anyone who wants the raw string.

Checks

• make python-test — 2963 passed
• make python-lint / make python-format / make python-typecheck / vulture — clean
• make frontend-test — 187 passed · frontend-lint / frontend-format / frontend-typecheck — clean

🤖 Generated with Claude Code

[calibrain]

Looks good to me ! Thank you !