Fix PDF width when exporting from split view

[PathGao]

Summary

• clear the interactive split-pane flex ratio in print media so PDF export uses the complete printable width
• add a regression test for the print-only rule

Root cause

The viewer pane receives an inline flex value in split view. The existing print CSS set width: 100%, but that does not override the flex-item sizing. PDF output from split view therefore used only the viewer's on-screen fraction of the page.

Validation

• npm ci
• npm run check
• npm test (150 passed)
• cargo test (27 passed)
• macOS isolated release bundle: exported a 20-section fixture from split view to a three-page A4 PDF; the result used full page width and each heading remained with its following paragraph

Fixes #328.

Chain

Depends on #327. Please merge after #327; it is intentionally a separate, print-only follow-up.

[PathGao]

@alecdotdev This is the next linear follow-up after #327. It fixes a confirmed macOS PDF regression specific to exporting while split view is active: the viewer pane retained its inline split flex ratio, so the PDF body used only part of the printable page. I verified the minimal print-only change in an isolated release bundle with a 20-section A4 export. Please merge it after #327; I left it as a draft until that dependency lands.