Improve duplicate detection and merging in moderation workflow

Overview: Make it easier to confirm and merge duplicates while moderating, without leaving the moderation screen. Related feature requests: Side-by-side comparison modal — the preferred solution for reliable duplicate confirmation with full context. Post #293707 Show similar posts in moderation — display candidates with full text visible, not just snippets. Post #293706 Working search field — quick search within moderation to find related posts. Post #293705 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, which isn't possible in today's moderation flow — so admins juggle tabs and re-search repeatedly. Benefit: Faster, more reliable duplicate detection and merging, all within the moderation screen, with enough context to make confident calls.