refactor(app): unified panel state machine for mobile sidebars

Replace separate sidebar stores with a unified panel store that uses a
discriminated union state machine on mobile. This makes it impossible
for both the agent list and file explorer sidebars to be open at the
same time on mobile.

Mobile state machine (mobileView):
- 'agent': Main agent view (no overlay panel)
- 'agent-list': Agent list sidebar (left overlay)
- 'file-explorer': File explorer sidebar (right overlay)

Desktop retains independent boolean toggles since sidebars sit
alongside content rather than overlaying it.

Key changes:
- Created stores/panel-store.ts with unified state
- Deleted stores/sidebar-store.ts and explorer-sidebar-store.ts
- Updated all components to use new panel store
- Animation contexts derive isOpen from unified state

packages/app/e2e/create-agent.spec.ts

@@ -4,13 +4,20 @@ import { createTempGitRepo } from './helpers/workspace';
 
 test('create agent in a temp repo', async ({ page }) => {
   const repo = await createTempGitRepo();
-  const message = `E2E create agent ${Date.now()}`;
+  const prompt = "Respond with exactly: Hello";
 
   try {
     await gotoHome(page);
     await setWorkingDirectory(page, repo.path);
     await ensureHostSelected(page);
-    await createAgent(page, message);
+    await createAgent(page, prompt);
+
+    // Verify user message is shown in the stream
+    await expect(page.getByText(prompt, { exact: true })).toBeVisible();
+
+    // Wait for agent response containing "Hello" within an assistant message
+    const assistantMessage = page.getByTestId('assistant-message').filter({ hasText: 'Hello' });
+    await expect(assistantMessage).toBeVisible({ timeout: 30000 });
   } finally {
     await repo.cleanup();
   }

packages/app/e2e/helpers/app.ts

@@ -41,14 +41,10 @@ export const ensureHostSelected = async (page: Page) => {
   if (await selectHost.isVisible()) {
     await selectHost.click();
 
+    // Wait for the host option to appear and click it
     const hostOption = page.getByText('localhost', { exact: true }).first();
-    if (await hostOption.isVisible()) {
-      await hostOption.click();
-    } else {
-      const fallbackOption = page.getByText('Local Host', { exact: true }).first();
-      await expect(fallbackOption).toBeVisible();
-      await fallbackOption.click();
-    }
+    await expect(hostOption).toBeVisible();
+    await hostOption.click();
   }
 
   await expect(input).toBeEditable();

packages/app/e2e/preserve-prompt-on-error.spec.ts

@@ -0,0 +1,25 @@
+import { test, expect } from './fixtures';
+import { gotoHome, ensureHostSelected, setWorkingDirectory } from './helpers/app';
+
+test('preserves prompt text when trying to create agent with non-existent directory', async ({ page }) => {
+  const nonExistentDir = '/non/existent/directory/that/does/not/exist';
+  const promptText = `Test prompt that should be preserved ${Date.now()}`;
+
+  await gotoHome(page);
+  await ensureHostSelected(page);
+  await setWorkingDirectory(page, nonExistentDir);
+
+  // Enter prompt text
+  const input = page.getByRole('textbox', { name: 'Message agent...' });
+  await expect(input).toBeEditable();
+  await input.fill(promptText);
+
+  // Try to submit - this should fail with an error about the directory not existing
+  await input.press('Enter');
+
+  // Verify error message is displayed (error includes the path)
+  await expect(page.getByText(/Working directory does not exist/)).toBeVisible();
+
+  // Verify the prompt text is still preserved in the input
+  await expect(input).toHaveValue(promptText);
+});