fix: restore horizontal scroll for wide tables in preview

[legolev]

Problem

Wide tables in the markdown preview run off the right edge and the extra columns are unreachable — there is no horizontal scrollbar, and window zoom doesn't help (it scales everything proportionally, so the overflow ratio stays the same). Most visible in a narrow window, in split view, and with CSV tables. Reported on Windows.

Root cause

The preview article (.markdown-body) is overflow-x: hidden by design. Since a5fda8e the table uses:

.markdown-body table { display: table; table-layout: fixed; width: 100%; }

table-layout: fixed; width: 100% forces every column to share the pane width. When it can't compress any further — the combined cell padding: 6px 13px (26px/cell) across many columns exceeds the available width, or a cell can't wrap (e.g. .csv-data cells are white-space: nowrap) — the table overflows and is silently clipped by overflow-x: hidden instead of scrolling.

Fix

Make the table a horizontal scroll container, the way GitHub's markdown CSS does:

.markdown-body table {
    display: block;
    width: max-content;
    max-width: 100%;
    overflow-x: auto;
    overflow-y: hidden;
}

The existing per-cell wrap rules (white-space: normal; overflow-wrap: anywhere) are kept, so tables whose content fits still wrap and never show a scrollbar — only genuinely wide tables scroll. A matching @media print override restores table-layout: fixed for PDF/HTML export, so exported tables wrap to fit the page (paper can't scroll).

Notes / trade-off

• Tables that fit now shrink to their content width instead of always stretching to 100% of the pane — this matches GitHub and most markdown renderers.
• Diff is 21 lines in src/styles.css, no markup or JS changes, so data-sourcepos on tables (scroll-sync) is untouched.

Testing

• npm run build — passes.
• npm run test:workflows — 25/25 pass (includes the HTML-export tests).
• Verified the layout in a Chromium/WebView2-equivalent harness: with the current CSS a wide table is clipped with no scroll; with the fix it scrolls and keeps natural column widths, while text-wrapping tables stay scrollbar-free. PDF (window.print() → @media print) still wraps.