Dark: added a prefers-color-scheme: dark CSS block, so HTML export
adapts to the browser/OS theme it's later viewed in. PDF is baked at
export time so it can't respond live — instead the offscreen WKWebView
used for PDF rendering has its appearance explicitly set to
NSApp.effectiveAppearance, matching whatever mode the app is in right
now at export time.
Tables: the renderer had no table support at all (previously
documented as an intentional scope cut), so GFM pipe tables were
falling through to the plain-paragraph path and showing up as literal
"| --- | --- |" text. Added detection (a row line immediately followed
by a valid dashes/colons separator row) plus alignment parsing from
the separator's colons.