Fix inbox reply parent targeting

[thomaspblock]

Summary

Fixes Inbox replies from grouped thread activity so the bottom composer replies to the selected item's existing parent/root thread instead of nesting under the latest activity item.

Before this change, an Inbox row whose selected item was the latest nested reply defaulted parentEventId to item.id, creating an unintended child reply under the latest comment. Now the default reply target is derived from the selected feed item's thread tags:

• reply rows default to their reply parent tag;
• root/top-level rows still default to their own id;
• explicit per-message reply targets in the Inbox detail pane still override the default.

Tests

• node --import ./desktop/test-loader.mjs --experimental-strip-types --test desktop/src/features/home/lib/inboxViewHelpers.test.mjs
• pnpm --dir desktop typecheck
• pnpm --dir desktop check
• pnpm --dir desktop test (1099/1099)
• pnpm --dir desktop build
• pnpm --dir desktop exec playwright test tests/e2e/smoke.spec.ts --project=smoke --grep "inbox composer replies"

Screenshots

Posted inline in #buzz-bugs with the verification flow:

1. Selected Inbox item is a latest nested comment.
2. Reply sent from Inbox appears in the thread; E2E asserts the send payload used the parent comment id, not the latest comment id.