feat: theme preset bundles

[smur89]

Problem

Users dialling in a look adjust accent, font, columnRatio, imagePosition, and headerTextAlign together — five knobs that interact. Discovering a coherent combination is trial and error. Even after the palette presets land, font + spacing + layout choices remain unbundled.

Proposed solution

Export a themes dict of vetted preset bundles:

#let themes = (
  classic: (font: "Lato",       accent: palettes.teal,    columnRatio: 0.64, headerTextAlign: "left"),
  modern:  (font: "Inter",      accent: palettes.navy,    columnRatio: 0.50, headerTextAlign: "center"),
  minimal: (font: "Source Sans 3", accent: palettes.charcoal, columnRatio: 0.55),
)

Usage: callers spread a theme over their own overrides.

#alta(cv, preferences: themes.modern + (imageSize: 7em))

(Builds on the palettes preset issue.)

Alternatives considered

• Document themes in README — no in-editor autocomplete; users still have to copy / paste the right dict.