Add a dark-mode theme

[spinsirr]

Lore is light-only today — the Vercel/Geist palette (near-white #fafafa canvas, near-black ink, hairlines) lives as CSS custom properties in src/app/globals.css. Add a dark theme by overriding those --* variables under @media (prefers-color-scheme: dark) (or a toggle).

The graph node palette is separate — TYPE_COLORS in src/lib/colors.ts — and may want dark-tuned values for contrast on a dark canvas.

Acceptance

• Readable dark theme that keeps the Geist aesthetic.
• Graph, dashboard, and search are all legible in dark mode.
• Light mode unchanged.

Pointers

src/app/globals.css (the :root custom properties), src/lib/colors.ts.