feat(layouts): offer six more page fonts, previewed in their own type

[patriksimms]

Problem

Layouts could only use Inter or Source Serif 4, so every book looked the same. The picker also listed both names in the app's own UI font, so you had to apply a font and look at the page to see what it actually was.

Solution

Six more families are available for every text element, grouped by category, and each name is rendered in the font it names. Everything is bundled with the app, so no font is ever fetched from Google at runtime.

Before	After

Picking a family carries through the editor canvas, the book preview and the exported PDF:

How it works

• src/domain/fonts.ts is now the single registry behind the picker, the browser preview, the layout metrics and the PDF cuts, so a family cannot render one way on screen and another in print.
• scripts/build-print-fonts.py (via bun run fonts:build) derives the static print TTFs and the web woff2 cuts from the pinned OFL variable sources on github.com/google/fonts. Only the derived files are served.
• Exports now embed just the cuts a book uses instead of all of them, so a typical export got smaller rather than larger despite four times as many fonts.
• Caveat has no italic cut upstream, so italic is disabled for it instead of silently rendering upright text.

Notable side effect

Routing families through the registry quotes the CSS family name. font-family: Source Serif 4 Variable unquoted is an invalid identifier and was silently dropped, so Source Serif 4 text previewed in Inter while the PDF printed it correctly. Preview and print now agree.

Trade-off

Advance-width metrics for 30 cuts grow the organizer-only layout editor chunk from about 159 kB to 221 kB gzip. Keeping them exact is what makes preview and PDF line breaking identical; trimming to a codepoint subset is possible later if that chunk becomes a problem.

Validation

bun run verify locally: format, lint, typecheck, 233 unit tests, 20 e2e tests, build. New coverage: registry-to-asset-to-metrics integrity, the Caveat italic fallback, and that a PDF embeds only the cuts in use. A rasterised export was inspected for each new family, umlauts and eszett included.

Closes #65

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Changes authored by Claude Opus 5 in Claude Code.

[patriksimms]

CI note: verify passed in full for 349c04a via the pull_request trigger (run 32230301297) — every step green, including the Linux e2e run and the layout parity snapshots.

The duplicate push-triggered run for the same commit (32230298722) is wedged on bunx playwright install --with-deps chromium, a step that took seconds in the run that passed. It does not respond to cancel, so it is a stuck runner rather than anything in this branch.