New card theme: high-contrast (accessibility)

[dchaudhari7177]

Every current theme is an aesthetic palette; none targets accessibility. This adds high-contrast, built to a contrast budget rather than a mood.

Measured ratios

Every text token clears WCAG AAA (7:1) against both the page background and the stat panel — not just against bg, since the big numbers sit on panel:

token	hex	vs bg #000000	vs panel #1c1c1c
fg	#ffffff	21.00 : 1	17.04 : 1
muted	#d0d0d0	13.62 : 1	11.05 : 1
accent	#ffd60a	14.88 : 1	12.07 : 1

The amber accent is deliberate: it stays distinguishable under the common red-green colour-vision deficiencies, which a green or red accent would not.

Test

Per the issue, the test asserts the computed contrast ratio, not just hex format. It adds a small dependency-free WCAG 2.1 luminance/ratio helper (matching the project's zero-dep stance), and:

• every text token is >= 7:1 against bg and panel;
• high-contrast is verifiably the highest-contrast theme registered — a regression guard if someone later tweaks it;
• the helper itself is checked against the WCAG reference values (white/black = 21:1, identical colours = 1:1), so a broken helper can't silently pass the theme.

Typecheck and the full suite are green.

Closes #35

[notsointresting]

Conflict against main now — a few other PRs touching src/cli.ts/themes/index.ts landed first. Could you rebase onto main and resolve? Should just be re-applying your change alongside the others already merged.