fix(print): use native controls and full page width

[PathGao]

Summary

• restore the macOS/Linux native print sheet through Tauri WebviewWindow::print(); the browser-level window.print() path can silently do nothing in the bundled app
• export directly to a user-chosen file through WebView2 on Windows, with print headers and footers disabled so PDFs do not contain tauri.localhost
• retain the full-width print layout when exporting from split view
• surface PDF-export failures as an in-app toast and bound the Windows completion wait to 60 seconds

Platform behavior

• macOS/Linux: Export as PDF opens the native print sheet, including the platform's existing “Save as PDF…” flow.
• Windows: Export as PDF opens a PDF save dialog, then uses WebView2 PrintToPdf with headers/footers disabled.

Validation

• npm ci
• npm run check — 0 errors, 0 warnings
• npm test — 128 passing
• cargo test
• cargo check --target x86_64-pc-windows-msvc
• built an isolated macOS release and exported the two reporter-provided stress documents from split view: the easy document was 29 A4 pages and the hard document was 92 A4 pages; inspected first, continuation, and table pages for full-width content and no URL footer.

Windows WebView2 output is compile-validated here; visual verification still needs a Windows runtime.

Related: #232

Depends on #337 until its test-only commit is merged.

[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.