fix(thread): render blocked-note placeholder and continue subtree walk

[dmnyc]

Summary

• In `buildNestedReplies`, a blocked note was silently dropped via `continue` with no further walk into its children
• This caused any replies nested under a blocked note — including the user's own — to disappear from the thread entirely
• Fix: mirrors the existing WoT-hidden behaviour — append the blocked placeholder row and `walk()` into children, so the subtree is preserved

Root cause

```swift
// Before — entire subtree silently dropped
if blockedEventIds.contains(kid.id) { continue }

// After — placeholder rendered, walk continues
if blockedEventIds.contains(kid.id) {
result.append(NestedReplyRow(row: makeRow(kid), depth: depth))
walk(parentId: kid.id, depth: depth + 1)
continue
}
```

The direct-reply list already kept blocked rows (`if row.isBlocked { visible.append(row) }`); nested replies now follow the same rule.

Before / After

Before	After

Test

Open a thread where a blocked user has a reply and you have a reply to that blocked note — your reply should now appear beneath the "Post from blocked user" placeholder instead of being hidden.

[dmnyc]

Follow-on to #380 — that PR introduced the WoT-hidden placeholder pattern; this applies the same walk-and-continue logic to blocked notes, which were previously dropped silently along with their subtrees.