[ivorb]

Follow-up to #3 (merged before this commit landed).

• "Your decks" section on the main page: decks saved in the browser get their own cards (title, file, slide count, date) with Open and Remove. The section only appears when something is saved.
• Open goes to convert.html?open=<id>, which converts the saved deck again and shows the preview.
• The converter page no longer has its own saved list or the browser-only note; after saving it links to the main page. Storage code is shared in assets/saved-decks.mjs.
• Title defaults to the file name when the deck has no title; the page scrollbar is hidden while the preview is open.

Checked

Browser test with a 15 MB, 44-slide deck: save, it appears under Your decks, Open shows the 44-slide preview, Remove deletes it and hides the section. node --test: 37 pass; manifest check clean.