Trim page props and page the conversation's messages

Bot and workspace lists in page props leave out instructions, memory and
workspace context, and carry the last message as a short preview. The
header avatar's edit form loads the bot's instructions when it opens.
The conversation renders its latest 30 messages and loads older ones as
the thread scrolls to its top.

Also: the composer shows a placeholder until its editor loads, and
`bun start` no longer builds first.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/data/app-data.ts

@@ -297,6 +297,20 @@ export const AppData = {
      * How much of the conversation a bot sees when it replies.
      */
     BotReplyHistoryMessages: 40,
+    /**
+     * How many of a session's latest messages the conversation page
+     * renders with; older ones load as the thread is scrolled up.
+     */
+    ConversationFirstMessages: 30,
+    /**
+     * How many older messages each scroll to the top of the thread loads.
+     */
+    ConversationOlderMessages: 30,
+    /**
+     * How much of a bot's last message the rail's lists carry, as plain
+     * text (`messagePreview`): a row shows one line of it.
+     */
+    RailPreviewMaxChars: 160,
     /**
      * A session with this many messages is closed when the user writes
      * again, and the message starts the next one (carrying a summary).