fix(web): enforce AA contrast for extended terminal colors

[lemkyz]

This is a focused follow-up to burakgon#89.

Why

The light palette covers ANSI colors 0–15, but the captured Claude terminal frame also uses xterm 256-color indexes 153, 246, 244, and 211. Against the paper terminal background, those colors measure approximately 1.39:1, 2.81:1, 3.65:1, and 2.08:1. Arbitrary truecolor output can bypass the 16-color palette in the same way.

What changed

• Add an optional minimumContrast value to GhosttyTerminalTheme.
• Enforce it in the canvas renderer against each cell’s actual foreground and background after inverse and selection handling.
• Account for faint text after alpha compositing.
• Parse both hex theme colors and the rgb() / rgba() strings emitted by the Ghostty runtime.
• Enable WCAG AA 4.5:1 enforcement only for light mode; dark and OLED retain their existing rendering.
• Keep live theme switching through the existing setTheme / rc-theme-change path.
• Use the same black-or-white fallback behavior as native Ghostty minimum contrast.

Verification

• 87/87 affected tests pass.
• pnpm typecheck passes.
• Scoped ESLint and git diff --check pass.
• Verified in the real screenshot.html?scene=terminal canvas: previously unreadable indexed and faint colors become legible immediately in light mode.

This leaves the picker, storage, theme-color meta, tokens, and base light ANSI palette from burakgon#89 unchanged.