fix(pdf): improve export pagination (v2.50.1)

[adibhanna]

Desktop PDF exports could strand headings at page bottoms and push images onto a new page despite leaving a large usable gap. Themed exports also lost their top and bottom content insets on continuation pages.

Keep headings with their following content, use repeating page margins with the theme background, and fit standalone images proportionally into the space available for the image and caption. Preserve small image sizes and explicit page breaks. Bump all eight package manifests and lockfile metadata to 2.50.1 without dependency changes.

Validation:

• npm run build:prod: passed all seven typecheck tasks, 4,391 JavaScript tests (five skipped), Go test suites, desktop/web/server builds, and isolated packaged CLI validation.
• npm audit --omit=dev --audit-level=high: no vulnerabilities.
• Actual Electron PDF regression exports passed for light and dark themes, including successive images, captions, forced breaks, image geometry, and paragraph continuity. All rendered pages were visually inspected on macOS.
• The original sample now fits its landscape image on page two and exports to five pages instead of six.
• git diff --check: passed.