refactor(app): replace 4 component test slop files with pure module extractions (#724)

Delete message-input.test.tsx, left-sidebar.test.tsx,
agent-stream-view.test.tsx, and agent-panel.test.tsx — all heavy
vi.mock + JSDOM + toHaveBeenCalledWith slop.

Coverage preserved by extracting the testable derivations:

- agent-stream-view-data.ts: isSameAssistantBlockGroup,
  getAssistantBlockSpacing, resolveInlineWorkingIndicatorItemId —
  14 pure unit tests, zero mocks
- message-input-state.ts: computeCanStartDictation (dictation readiness
  gate) — 7 pure unit tests, zero mocks

Remaining behaviors confirmed covered by existing E2E:
- left-sidebar subscription-while-hidden → sidebar-workspace.spec.ts
- agent-panel render isolation + archived agent store hygiene → archive-tab.spec.ts
- message-input attachment menu / submit icon → workspace-setup-streaming.spec.ts

packages/app/src/components/agent-stream-view-data.ts

@@ -0,0 +1,51 @@
+import type { StreamItem } from "@/types/stream";
+
+export function isSameAssistantBlockGroup(params: {
+  item: StreamItem | null | undefined;
+  other: StreamItem | null | undefined;
+}): boolean {
+  return (
+    params.item?.kind === "assistant_message" &&
+    params.other?.kind === "assistant_message" &&
+    params.item.blockGroupId !== undefined &&
+    params.item.blockGroupId === params.other.blockGroupId
+  );
+}
+
+export function getAssistantBlockSpacing(params: {
+  item: StreamItem;
+  aboveItem: StreamItem | null | undefined;
+  belowItem: StreamItem | null | undefined;
+}): "default" | "compactTop" | "compactBottom" | "compactBoth" {
+  if (params.item.kind !== "assistant_message") {
+    return "default";
+  }
+  const compactTop = isSameAssistantBlockGroup({ item: params.item, other: params.aboveItem });
+  const compactBottom = isSameAssistantBlockGroup({ item: params.item, other: params.belowItem });
+  if (compactTop && compactBottom) return "compactBoth";
+  if (compactTop) return "compactTop";
+  if (compactBottom) return "compactBottom";
+  return "default";
+}
+
+export interface NeighborResolver {
+  getNeighborItem(
+    items: StreamItem[],
+    index: number,
+    relation: "above" | "below",
+  ): StreamItem | undefined;
+}
+
+// null → auxiliary working indicator; non-null → inline footer on that block.
+export function resolveInlineWorkingIndicatorItemId(
+  status: string,
+  liveHeadItems: StreamItem[],
+  strategy: NeighborResolver,
+): string | null {
+  if (status !== "running") return null;
+  const footerItem = liveHeadItems.find((item, index, items) => {
+    if (item.kind !== "assistant_message") return false;
+    return strategy.getNeighborItem(items, index, "below") === undefined;
+  });
+  return footerItem?.id ?? null;
+}