tokimeter card: add a light and print theme

[toshipepe]

tokimeter card renders a shareable dark "My month in AI" SVG. Add a light variant that works better in slides and print.

What's needed:

• Add --theme=light to the card command.
• Parameterize the hard-coded SVG colors behind a small palette.
• Preserve the honest-numbers footer and the guarantee that cards contain no project paths or session IDs.
• Add tests showing the light palette is used while the disclaimer remains and no private metadata appears.

This is intentionally small and should not add dependencies.

[volkkov]

This behavior changed in v2.4 — the migration guide mentions it but it's easy to miss. Adding a note to the docs PR.