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/message-input-state.ts

@@ -0,0 +1,14 @@
+import type { DaemonClient } from "@server/client/daemon-client";
+
+export function computeCanStartDictation(input: {
+  client: DaemonClient | null;
+  isReadyForDictation: boolean | undefined;
+  disabled: boolean;
+  dictationUnavailableMessage: string | null | undefined;
+}): boolean {
+  const socketConnected = input.client?.isConnected ?? false;
+  const readyForDictation = input.isReadyForDictation ?? socketConnected;
+  return (
+    socketConnected && readyForDictation && !input.disabled && !input.dictationUnavailableMessage
+  );
+}