feat(desktop): import VS Code color themes

[lihuiyang1024]

Description

Closes #152.

Implements the bounded VS Code theme-import contract agreed in the issue: one local standalone JSON/JSONC color-theme file, no package discovery or include traversal, mapped into the existing Appearance theme system.

Behavior

• imported is a regular ThemePreference and uses data-theme="imported"
• every imported palette is complete: unmapped values are filled from an inferred light/dark base
• only the normalized 32-token palette, base, and display name are persisted; the path and raw source are discarded
• all colors values must use VS Code's hexadecimal color notation
• invalid JSONC, invalid colors, files over 1 MB, and include chains produce a visible error without replacing the current theme
• syntax tokenColors, extension packages, and include resolution are deliberately outside this first version

Changes Made

• add a pure JSONC parser/normalizer and documented VS Code-to-DeepCode token mapping
• derive translucent accent states and valid box-shadow declarations from imported colors
• add an import control, persisted imported-theme option, status, errors, and zh-CN copy to Settings → Appearance
• add a minimal imported-theme CSS selector through the existing theme mechanism
• pin the light/dark fallback copies to tokens.css in tests so they cannot drift or become incomplete
• document the local-only trust and persistence boundary in desktop/README.md

Verification

• npm run typecheck
• npm run lint
• focused import/appearance/token/UI tests — 96 passed
• npm test — 32 files, 207 tests passed
• npm run build

Checklist

• Changes tested locally
• Code reviewed
• Documentation updated
• Unit tests added