feat(export): Export to PDF — clean-copy print-to-PDF

[sam-powers]

Implements the Export to PDF for sharing backlog item (PRD §7): a one-shot export of the current document to PDF so it can be shared with people who don't have Quill.

Two design decisions, made with Sam up front:

• Clean copy only — the PDF shows the finished document (pending suggestions rendered as accepted; comment/track-changes markup left out). A review copy (markup visible) is recorded as a deliberate, revisitable non-goal in PRD §7.
• Webview print-to-PDF — reuse the editor's own DOM + typography through a print stylesheet and the OS print pipeline, rather than a second Markdown→HTML→PDF pipeline that would drift from the editor.

What's in it

• File → Export to PDF… (Cmd+P) — new native menu item emitting menu-export-pdf, wired through App.tsx's menu-handler ref alongside the other File items. The non-native-menu path (dev/e2e) handles Cmd+P too, so it works in npm run dev.
• handleExportPdf sets document.title to the document's name (so the OS dialog defaults the filename), calls window.print(), and restores the title after the synchronous dialog. On macOS the dialog's Save as PDF writes the file.
• @media print block in App.css defines the artifact:
  • Hides all chrome (toolbar, footer, comment layer, find bar, update banner, modals).
  • Collapses the on-screen height:100vh / overflow:hidden scroll + zoom containers so content reflows and paginates naturally on paper; resets the screen zoom and the dashed page-break guide lines.
  • Clean copy of the markup: ins.track-insert → plain text, del.track-delete → display:none, comment/find/annotation-focus highlights stripped.
  • US Letter, 0.75in margins; the OS owns pagination.

Docs

• PRD §3.5 documents the feature; §7 records review-copy export as a deliberate non-goal and drops the old "Export to PDF" backlog entry.
• USER_GUIDE gets a one-line "Export to PDF" entry under The Basics.

Verification

typecheck, eslint, prettier, vitest (255), cargo fmt/clippy/test (35), and vite build all green locally.

⚠️ The printed artifact itself can't be asserted in CI (it's the OS print pipeline + a print stylesheet). It needs a manual print-preview pass in the Tauri app — confirm chrome is gone, deletions vanish, insertions read as plain text, and pagination looks right — before merge.

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