fix(desktop): bound thread /query and surface load errors, not false-empty

[wpfleger96]

Long threads in the desktop app sometimes never load (stuck on a skeleton until you close and reopen the panel), and a failed load silently renders as "No replies in this branch yet" — presenting a broken fetch as an authoritative empty thread with no way to recover. This fixes both, the two IMPORTANT findings from the thread-load investigation.

Defect 1 — unbounded /query request

The shared reqwest::Client in relay.rs sets no timeout, and neither /query request builder set a per-request .timeout(...). A stalled or half-open connection (headers or body never arrive) leaves the request pending forever, so a thread-history load hangs on the skeleton indefinitely.

Fix: a 30s per-request deadline on both /query builders. Scoped per-request rather than client-level because the same client also serves STT/TTS model downloads, builderlab auth, and the media proxy — a client-level timeout would cut those off. The deadline sits above the 25s WS HISTORY_TIMEOUT_MS so a slow-but-live relay isn't cut off before the WebSocket path would be. A timeout surfaces through classify_request_error as the stable "relay unreachable: request timed out" string.

Defect 2 — terminal error painted as empty

ChannelScreen consumed only isPending/data from the thread-replies query. Once React Query exhausted its one retry, isPending was false and the zero-length data fell through selectDeferredListRenderState to the "empty" state — indistinguishable from a genuinely empty branch, with no retry affordance.

Fix: plumb isError + refetch through ChannelScreen → ChannelPane → MessageThreadPanel. A pure selectThreadRepliesSurface helper decides the paint in strict precedence — the load-bearing invariant is that a terminal error never resolves to "empty", and cached replies stay visible non-destructively under a later error (the error card only surfaces when there is nothing to show). The panel renders an explicit "Couldn't load replies" + Retry card (testids message-thread-replies-error / message-thread-replies-retry).

Tests

• stalled_query_request_times_out_with_classified_error — a loopback server that never responds; asserts is_timeout() and the stable classified string.
• selectThreadRepliesSurface — pending→skeleton, terminal error→error (never empty), page-2 failure never empty, cached rows stay visible under error, successful-empty→empty, retry-success→list, streaming→pending, and huddle-transcript collapse.
• selectThreadRowHighlight — branch-range membership, direct-child, and guide-suppression cases for the extracted highlight helper.

Structure

To stay under the desktop file-size ratchet (both relay.rs and MessageThreadPanel.tsx were one line under the 1000-line ceiling), relay.rs's inline test module moved to relay/tests.rs, and two pure pieces were extracted from the panel: the empty/error reply cards (MessageThreadReplyState) and the per-row branch-highlight derivation (selectThreadRowHighlight).