fix(app): synchronize chat submission and keyboard state

Render optimistic turn feedback before host acknowledgement and roll it back on rejection. Reconcile iOS keyboard offsets from the native transition end event so JS contention cannot leave the composer displaced.

packages/app/src/timeline/turn-time.test.ts

@@ -22,6 +22,36 @@ function assistant(id: string, timestamp: Date): StreamItem {
 }
 
 describe("deriveStreamTurnTiming", () => {
+  it("reserves a running footer for an optimistic prompt before the host starts the turn", () => {
+    const optimisticPrompt = {
+      ...user("optimistic", new Date("2026-05-15T00:00:00.000Z")),
+      optimistic: true as const,
+    };
+
+    const timing = deriveStreamTurnTiming({
+      agentStatus: "idle",
+      tail: [],
+      head: [optimisticPrompt],
+    });
+
+    assert.equal(timing.isActive, true);
+  });
+
+  it("does not start elapsed time from an optimistic prompt", () => {
+    const optimisticPrompt = {
+      ...user("optimistic", new Date("2026-05-15T00:00:00.000Z")),
+      optimistic: true as const,
+    };
+
+    const timing = deriveStreamTurnTiming({
+      agentStatus: "running",
+      tail: [],
+      head: [optimisticPrompt],
+    });
+
+    assert.equal(timing.runningStartedAt, null);
+  });
+
   it("uses the last user message as the running turn start", () => {
     const firstUserAt = new Date("2026-05-15T00:00:00.000Z");
     const secondUserAt = new Date("2026-05-15T00:01:00.000Z");