feat: export themes preset bundles for coherent visual styles

[smur89]

Summary

Adds a module-scope themes dict (classic, modern, minimal) of vetted preset bundles — each themes the four knobs that interact visually (accent, font, columnRatio, headerTextAlign). Callers spread a theme over their own overrides:

#import "@preview/altacv:1.1.1": alta, themes // x-release-please-version
#alta(cv, preferences: themes.modern + (imageSize: 7em))

Closes #58. Supersedes #70 (PR predates the lib.typ → internal//sections/ split + the internal/labels-en.toml extraction).

Design notes

• classic re-declares current defaults (including columnRatio: 0.65, matching _default_preferences) — themes are by-name bundles, not pointers to "whatever the default is right now". Defaults can shift without silently shifting classic.
• imagePosition is intentionally not bundled: it only makes sense when basics.image is set; baking it would either force image-only assumptions or no-op for image-less CVs.
• Themes reference palettes.* for accents, so any future palette tweak ripples through.
• modern (Inter) and minimal (Source Sans 3) fonts are aspirational. Neither is on CI. The fixture overrides font: "Lato" for those themes so make test stays silent; users with the named font installed get the intended typography, others get Typst's standard font-fallback warning.

Test plan

• make test — tests/themes.typ and all pre-existing fixtures green.
• tests/themes.typ iterates themes.keys() so a future theme auto-extends the fixture; assert.eq pins each theme's declared font so a typo ("Intr") is caught even though the CI-host fallback would mask it visually.
• Tracked examples/tests/themes.pdf.
• README updated with a Theme presets subsection (table + spread-and-override example).

Summary by CodeRabbit

• New Features

  • Added three built-in theme presets: classic, modern, and minimal, each combining font, accent colour, and layout-related styling.
  • Presets can be composed with your own overrides for a tailored appearance.

• Documentation

  • Updated the README with usage guidance for theme presets, including how preset values apply and how unspecified settings fall back to defaults.
  • Expanded the “Helpers” examples to include the new themes helper alongside existing helpers.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 5dca15f7-e360-432f-8bef-7ccc91a14a63

📥 Commits

Reviewing files that changed from the base of the PR and between 74e677a and ea77820.

⛔ Files ignored due to path filters (1)

• examples/tests/themes.pdf is excluded by !**/*.pdf

📒 Files selected for processing (4)

• README.md
• internal/presets.typ
• lib.typ
• tests/themes.typ

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📝 Walkthrough

Walkthrough

A themes preset bundle (classic, modern, minimal) is added to internal/presets.typ, each defining font, accent, columnRatio, and headerTextAlign. It is re-exported from lib.typ, exercised in a new tests/themes.typ fixture, and documented in README.md.

Changes

Theme Presets Bundle

Layer / File(s)	Summary
themes preset definition and re-export internal/presets.typ, lib.typ	Adds the themes dict with classic, modern, and minimal entries — each setting font, accent (via palettes), columnRatio, and headerTextAlign. Updates the lib.typ import to include themes alongside existing presets.
Test fixture tests/themes.typ	New fixture renders a sample CV four times: once per built-in theme and once demonstrating the spread-and-override pattern (themes.modern with font and imageSize overrides). Includes font assertions and CI font workaround.
README documentation README.md	Adds a "Theme presets" subsection covering the three presets, the spread syntax, and font-fallback behaviour. Extends the Helpers table and import snippet to include themes.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

• smur89/alta-typst#79: The new themes presets reference palettes entries (teal, navy, charcoal) directly, making this a direct continuation of the palette preset infrastructure introduced in that PR.

Poem

🐇 Three themes hopped in, all tidy and bright,
Classic, Modern, Minimal — each set just right.
Spread them and override with a flick of a key,
columnRatio, accent, and font all agree.
No more trial and error at the end of the day —
The rabbit has bundled the knobs away! 🎨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarises the main change: exporting a themes preset bundle for visual styles.
Description check	✅ Passed	The description is comprehensive and follows the template with all key sections filled: summary, design notes, and test plan.
Linked Issues check	✅ Passed	All coding requirements from issue #58 are met: themes dict with classic/modern/minimal presets, spreading syntax support, palette integration, and documentation.
Out of Scope Changes check	✅ Passed	All changes directly support the themes preset feature: presets file, library exports, tests, and README documentation.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/issue-58-theme-presets-v2

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