fix(web): preserve imported theme colors and contrast

[asfires]

What Changed

• Preserve imported VS Code theme foreground, status, surface, hover, border, and accent colors when generating T3 palettes.
• Treat fully transparent theme colors as unset and fall back to visible candidates.
• Derive readable text and status foregrounds by adjusting lightness while retaining the source hue.
• Keep surface ramps and borders visually distinct without flattening the imported palette.
• Expand Open VSX and VS Code theme import coverage with focused regression tests.

Why

Imported themes could lose their visual identity during palette generation: muted text became too bright, surfaces collapsed into the canvas, transparent values erased hover and focus states, and status colors could fall back to unrelated defaults. This update keeps the theme’s original color signals while ensuring the resulting palette remains readable and visually distinct.

UI Changes

No screenshots included; the changes affect generated theme colors and contrast across imported themes rather than a specific layout.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes