feat(ST-1110.5): unify demo write-button pattern to disabled+tooltip

Convert all !isDemo && <Button> patterns to <DemoTooltip show={isDemo}>
<Button disabled={isDemo}> so demo visitors see app capabilities.
Affects: pbi-list, story-panel, story-dialog, task-list, sprint-backlog,
token-manager, product-list, activate-product-button, leave-product-button,
settings page.

__tests__/api/pair-start.test.ts

@@ -1,7 +1,12 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest'
 
-const { cookieJar } = vi.hoisted(() => ({
+const { cookieJar, mockGetIronSession } = vi.hoisted(() => ({
   cookieJar: { set: vi.fn(), get: vi.fn(), delete: vi.fn() },
+  mockGetIronSession: vi.fn().mockResolvedValue({ isDemo: false }),
+}))
+
+vi.mock('iron-session', () => ({
+  getIronSession: mockGetIronSession,
 }))
 
 vi.mock('@/lib/prisma', () => ({