Export dialog: remember last-used page range

[Kayforkind]

The export dialog resets the page range to 'all pages' every time it opens. Remember the user's last choice in localStorage (same pattern as the existing saved-session key) and pre-select it on open.

Scope: one small state addition in the export dialog component + read/write to localStorage.

[Kayforkind]

Done — the export dialog now remembers the last-used page range in localStorage (pdfstudio.export.range) and pre-selects it when reopened. It's saved when you click Export; Split doesn't save it since splitting ignores the range.