Add "themeName": "auto" to follow the app theme

[akurach]

Implements #167.

With "themeName": "auto", the preview resolves its theme from the app's current theme (MarkEdit.editorConfig.theme) instead of a fixed name:

• The -light/-dark/-dawn suffix is stripped to get the preview theme name — every built-in app theme maps onto an existing preview theme, so no new CSS is needed.
• The suffix also pins the color scheme (applied in coreCss/previewThemeCss/alertsCss/hljsCss), so xcode-dark keeps the preview dark under a light system appearance. Suffix-less app themes (dracula, cobalt, night-owl, synthwave84, rose-pine) are treated as dark. rose-pine-dawn maps to rose-pine light.
• Since the app theme can change at runtime and there is no change notification for extensions, the theme/hljs style sheets are refreshed on each preview render and (with a small delay, matching the app's own switch) on prefers-color-scheme changes. Explicitly configured theme names behave exactly as before.
• Search-match highlight colors follow the resolved theme as well.

Default behavior is unchanged: themeName still defaults to github, "none" still renders raw HTML.

Tests: added tests/theme.auto.test.ts covering the mapping (suffixed, suffix-less, rose-pine-dawn, missing config). yarn lint passes; the full test suite shows the same 15 pre-existing failures as main in my environment (Node 26, localStorage-related), with no new ones.

[cyanzhong]

The way we refresh doesn't seem to be optimal, I can add a new onEditorThemeChange event to markedit-api so we safely refresh the theme.

[akurach]

Agreed — an official onEditorThemeChange in markedit-api would be much cleaner than refreshing on render. If you add it, I'll switch this PR to subscribe to the event (keeping the render-time refresh only as a fallback for older app versions, or dropping it entirely if you prefer a minimum-version requirement).

[cyanzhong]

@akurach I've added onEditorConfigChange: MarkEdit-app/MarkEdit-api@501a167 The main branch of this repo integrated the new types. MarkEdit.editorConfig is also strictly typed.