Color palette system

[joshcoolman]

Formalize and extend the existing card color palette (CARD_THEMES in lib/card-themes.ts) into a first-class system.

What exists now:

• 11 hand-curated CardTheme entries (bg / fg / muted / accent) cycling across all card surfaces
• A named source palette (C) of ~20 hex values the themes are built from
• PAGE_THEME for chrome and HIGHLIGHT for active controls — both fixed

Direction:

• Define the palette system more explicitly — named ramps, roles, relationships between colors
• Make it possible to add new themes without breaking the existing rotation
• Potentially expose controls for extending or customizing the palette

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Migrated from BACKLOG.md.

[joshcoolman]

Largely superseded by #17 (User-editable card themes), which specifies concretely what this issue framed as direction: adding themes without breaking the rotation (copy-on-edit, built-ins index-stable), and exposing controls for customizing the palette (the editor modal + user-theme store).

What #17 does not cover from this issue: defining named ramps, roles, and relationships between the ~20 source hex values in C, and making PAGE_THEME / HIGHLIGHT non-fixed.

Those two remain live — but they now belong with #19, which ports the token set to src/styles/tokens.css and is where role-naming decisions get made. Suggest closing this and letting #17 + #19 carry it.