Side-by-side comparison modal for duplicate detection in moderation

Description: Add a side-by-side comparison modal in the moderation screen: Pane A = the post being moderated Pane B = a candidate similar post shown in full Ability to scroll through the 4–5 most similar candidates Merge directly from the modal Problem: Checking for duplicates while moderating is currently manual: you open a separate tab, search, and can't merge until the first post is approved. Reliable duplicate confirmation needs the full text of both posts visible at once. Benefit: Faster, more reliable duplicate detection and merging, all within the moderation screen, with enough context (full text, side by side) to make confident calls.