feat: message-per-row transcript, session resume, drop the composer (#4)

src/lib/thread.ts

@@ -0,0 +1,72 @@
+import { previewText } from "./markdown";
+import type { ChatTurn, ThreadMessage } from "./types";
+
+/**
+ * Flattens stored turns into the flat message list the transcript renders.
+ *
+ * Storage keeps a question and its answer together as a turn, because that is
+ * what gets replayed to the model. The UI needs the opposite shape: a flat
+ * sequence so the user's line and Lam's line can each carry their own icon and
+ * weight. This is the one place that translation happens.
+ */
+
+/** Rows are a single line, so previews are clipped well before the renderer elides them. */
+export const MESSAGE_PREVIEW_LENGTH = 140;
+
+export interface LiveState {
+  isStreaming: boolean;
+  /** Partial answer for the turn currently in flight. */
+  response: string;
+  thinking: string;
+  /** Activity line, e.g. "Searching the web". */
+  status?: string;
+}
+
+export function buildThread(turns: ChatTurn[], live: LiveState): ThreadMessage[] {
+  const messages: ThreadMessage[] = [];
+  const lastIndex = turns.length - 1;
+
+  turns.forEach((turn, index) => {
+    messages.push({
+      id: `${turn.id}:user`,
+      role: "user",
+      text: turn.prompt,
+      preview: previewText(turn.prompt, MESSAGE_PREVIEW_LENGTH),
+      createdAt: turn.createdAt,
+      isStreaming: false,
+      thinking: "",
+      toolCalls: [],
+    });
+
+    // Only the newest turn can be mid-flight.
+    const isLive = index === lastIndex && live.isStreaming;
+    const text = isLive ? live.response : turn.response;
+
+    messages.push({
+      id: `${turn.id}:assistant`,
+      role: "assistant",
+      text,
+      preview: assistantPreview(text, isLive, live.status, turn.error),
+      createdAt: turn.createdAt,
+      isStreaming: isLive,
+      thinking: isLive ? live.thinking : turn.thinking,
+      toolCalls: turn.toolCalls,
+      error: isLive ? undefined : turn.error,
+    });
+  });
+
+  return messages;
+}
+
+/**
+ * The row label for an answer.
+ *
+ * Before the first token lands there is nothing to preview, so the activity
+ * line stands in — otherwise the row would sit blank while the model works.
+ */
+function assistantPreview(text: string, isLive: boolean, status: string | undefined, error?: string): string {
+  if (text.trim().length > 0) return previewText(text, MESSAGE_PREVIEW_LENGTH);
+  if (isLive) return status ?? "Thinking…";
+  if (error) return error;
+  return "No response";
+}