fix: allow independent light/dark theme selection when following system

[xielixing]

Summary

When "Match system" is selected as the theme, users can now independently choose which light theme and which dark theme the system mode uses. Previously, system mode always fell back to the hardcoded defaults (bitfun-light / bitfun-dark).

Closes #1080

Changes

• presets/index.ts: getSystemPreferredDefaultThemeId() now accepts optional lightId/darkId override parameters, falling back to the built-in defaults when not provided.
• ThemeService.ts: Added systemLightId/systemDarkId fields with lazy loading from themes.systemLightId/themes.systemDarkId config paths. Added setSystemThemeOverride() method that persists overrides and re-resolves the active theme if system mode is active. All three call sites of getSystemPreferredDefaultThemeId() now pass the user-configured overrides.
• themeStore.ts: Added systemLightId/systemDarkId state and setSystemThemeOverride action that delegates to themeService.
• useTheme.ts: Exposed systemLightId, systemDarkId, and setSystemThemeOverride from the hook.
• AppearanceConfig.tsx: Added two conditional Select dropdowns (light themes / dark themes) that appear only when "Match system" is selected, letting users pick independent light/dark themes.
• i18n: Added 4 new keys to en-US, zh-CN, and zh-TW locale files.

Design decisions

• System theme overrides are loaded lazily — only when the user actually selects "Match system" mode. This avoids unnecessary config reads during initialization and preserves existing test expectations.
• Overrides are validated against the registered themes map; invalid IDs fall back to defaults silently.
• Config paths follow the existing themes.* namespace convention (themes.systemLightId, themes.systemDarkId).

Validation

• tsc --noEmit — 0 errors
• vitest run src/infrastructure/theme — 37/37 tests passed
• pnpm run i18n:audit — 0 warnings