Fix Webview theme adaptation

[SivanCola]

Summary

• derive Webview backgrounds, panels, borders, shadows, and subtle highlights from VS Code theme colors
• remove dark-only surface colors that made the chat view look gray and disconnected in light themes
• preserve Reasonix accents while deriving readable foreground variants from the active VS Code foreground
• add regression coverage for old dark-only colors and WCAG contrast across representative light, dark, and mid-tone themes

Root cause

The Webview already consumed some --vscode-* colors, but its shared Reasonix surface palette mixed fixed near-black colors and white highlights into those values. In light themes this darkened the entire chat surface and left several controls with dark-theme-specific foregrounds.

Validation

• npm run lint
• npm test (69 tests)
• npm audit --audit-level=high
• npm run smoke:acp
• npm run package:vsix
• npm run verify:vsix-contents
• interactive light/dark Webview preview with no console warnings or errors

The local macOS npm run test:vscode runner now resolves the VS Code executable after the base dependency update, but the local process is terminated by SIGKILL before test assertions start. Linux and Windows Extension Host paths remain covered by CI.

Closes #6