Show replies inside nested threads on mobile

[BradGroux]

Fixes #2415.

What mobile users saw

Opening a reply as the head of a nested thread showed no direct replies, even
when those replies existed on the relay.

Why it happened

ThreadDetailPage asked threadRepliesProvider for relay history using the
nested head ID. Relay thread history is keyed by the original root event, so the
query returned no history for that nested head.

The page already knew the effective root later in the build and already
filtered the result down to direct children. The history query was using the
wrong ID.

What changed

• Query root-scoped history with the original thread root.
• Keep the existing direct-child filter so the nested view still shows only
  replies to the selected head.
• Add a Flutter widget regression for root R → nested head M → direct reply
  C.

Regression proof

Before the production fix, the regression expected #e == [root] and received
[nested-head]. It passes with the root-scoped query.

Verified at commit 970bcc4bef40bdf86d0e10a73a030b0e6731a897.

Verification

• just mobile-check
• just mobile-test — 529 passed, 1 skipped
• just ci — Rust, Desktop, web, and mobile gates passed
• independent specification and standards reviews — no findings
• GitHub DCO, Semgrep, and zizmor — green