Missing features from old RedditLater

* [ ] Copy feature
* [ ] Markdown preview fails to handle ###-style headers
* [ ] Make post time field 24h (i.e. don't use time input)

Start by locating the existing copy feature, Markdown preview, and post-time field described in the checklist. Verify the current behavior for ###-style headers and time entry, then consider each checklist item complete only when copying works, headers render correctly, and post times use a 24-hour format.

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