fix(safety): show WoT-hidden thread replies as placeholders

[dmnyc]

Summary

• WoT-hidden replies in a thread now render as placeholder rows instead of being silently dropped
• A single hidden reply shows "Post hidden by WoT filter"; consecutive hidden replies at the same depth collapse into one row: "2 posts hidden by WoT filter", etc.
• No content, author, or media is revealed — the placeholder only preserves the thread's shape
• WoT enforcement is unchanged; the filter still runs as before

Screenshot

Test plan

• Open a thread on an account with WoT enabled — replies from outside WoT should show placeholder rows
• Verify multiple consecutive hidden replies at the same depth collapse to a count row ("2 posts hidden by WoT filter")
• Verify WoT-hidden replies at different depths each get their own placeholder
• Confirm visible replies from WoT-qualified users are unaffected
• Toggle WoT off — all replies should render normally with no placeholders
• Confirm no content, author, or media leaks through the placeholder

Closes #379