fix: wire orphaned IPC channels, fix data-loss bug, print preview events, sidebar navigation

- Fix file.clearRecent sending IPC to main process instead of clearing openTabs
- Wire show-batch-converter, show-document-compare, open-header-footer-dialog IPC channels
- Add print preview event listeners (mc:print-preview, mc:print-preview-styled) in App.tsx
- Fix sidebar hidden bridge buttons toggling sidebar closed after scroll
- Update sidebar menu labels to match actual sections (Files, Outline, Git)
- Fix ipc.print to be an object with show/doPrint methods (was bare function)
- Update callers: ExportPdfDialog, pdf-export, PrintPreview
- Add 7 regression tests for all fixes
- Clean 2 obsolete snapshots
- Build Linux packages (deb, AppImage, snap) — 549 tests passing

src/renderer/components/modals/ExportPdfDialog.tsx

@@ -51,7 +51,7 @@ export function ExportPdfDialog({ sourcePath }: { sourcePath: string }) {
       const m = MARGIN_MAP[margins];
       const pageCss = `@page { size: ${fmt.width} ${fmt.height}; margin: ${m.top}mm ${m.right}mm ${m.bottom}mm ${m.left}mm; }`;
       const finalHtml = html.replace('</style>', `${pageCss}</style>`);
-      const result = await ipc.print({ html: finalHtml, withStyles: embed });
+      const result = await ipc.print.show({ html: finalHtml, withStyles: embed });
       if (!result.ok) {
         const msg = result.error?.message ?? 'PDF export failed';
         toast.error(`Export failed: ${msg}`);