ctx.ui.setTitle(undefined) from an extension crashes the whole page #1356

Description

When a pi extension calls ctx.ui.setTitle(undefined) or ctx.ui.setTitle(null), the whole page is replaced by "The app panel hit an error", not only the chat panel. Only a page reload brings the app back.

pi types the argument as string, but pi loads extensions without a type check: my .ts test extension passes undefined and loads without errors. One wrong call from any extension breaks the whole UI.

How it happens (line numbers are from main at 52ae5478):

1. setTitle publishes the value as it is (packages/server/src/agent/webUiContext.ts:214-216). The host sends the frame with JSON.stringify (packages/server/src/host/server.ts:1160), so for undefined the frame has no title field at all, and for null it has "title": null. The contract says title: string (packages/contracts/src/piProtocol.ts:309).
2. The web takes the frame without checks (apps/web/src/transport/wireTransport.ts:393), and applyExtUi passes request.title straight to renameChat (apps/web/src/store/appStore.ts:4724-4725). The chat tab gets name: undefined. The session_info_changed path uses the same renameChat, but it normalizes the name first: event.name?.trim() ?? "" (appStore.ts:4593). The store spec says this shared rename should "normalize an absent name to the empty untitled name" (apps/web/src/store/SPEC.md:253-255). But the normalization is in the session_info_changed caller, not in renameChat, so the setTitle path skips it.
3. selectUnnamedChatFirstPrompts calls tab.name.trim() (apps/web/src/store/selectors.ts:1276). It runs in WorkspaceWorkbench (apps/web/src/shell/WorkspaceWorkbench.tsx:612). No closer error boundary catches the TypeError, so the app-level one (apps/web/src/main.tsx:51) does and replaces the whole page.

An idea for the fix: the host could always send a string ("" for a missing title), so the frame matches the contract. And applyExtUi could normalize the title the same way as session_info_changed.

Found while working on #1353. That PR keeps this behavior: its stripAnsi returns non-strings unchanged, and its test expects a setTitle frame with title: undefined.

Steps to reproduce

1. Add a global extension, for example ~/.pi/agent/extensions/title-undefined.ts:

   export default function (pi) {
   	pi.registerCommand("title-undefined", {
   		description: "calls ctx.ui.setTitle(undefined)",
   		handler: async (_args, ctx) => {
   			ctx.ui.setTitle(undefined);
   		},
   	});
   }

2. Start ThinkRail with bun run dev, open a project and start a chat. No provider is needed: an extension command doesn't call the model.

3. Send /title-undefined in the chat.

Expected behavior

The app keeps working. The chat tab falls back to its untitled name, the same as when pi reports a session without a name.

Actual behavior

The whole page shows "The app panel hit an error" with Cannot read properties of undefined (reading 'trim'). "Try again" shows the same error. A page reload brings the app back.

The console has the TypeError from selectUnnamedChatFirstPrompts, called from WorkspaceWorkbench, and [ErrorBoundary: app] reports it.

ctx.ui.setTitle(null) does the same, with Cannot read properties of null (reading 'trim'). ctx.ui.setTitle("Title from extension") renames the tab as expected.

Environment

• OS: macOS 27.0.1
• Bun version (bun --version): 1.4.4
• Node version (node --version): v24.10.0
• ThinkRail version / commit: main 52ae5478, pi 1.5.2, web UI from bun run dev in a Chromium-based browser