Migrate popup chat window from vanilla HTML to React

[sbaker]

Summary

Replace popup-input.html with a React-based popup using @prompd/react chat components.

Why

@prompd/react already has PrompdChat, PrompdMessages, PrompdChatInput with markdown rendering, streaming, and theming. The current vanilla HTML implementation duplicates these patterns. Migrating to React unifies the codebase and lets the popup benefit from shared component improvements automatically.

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Sub-tasks

• Create frontend/src/popup.tsx React entry point
• Create frontend/popup.html shell
• Update vite.config.ts with multi-page entry
• Create PopupChat component using @prompd/react
• Support all input types: text, textarea, number, choice, confirm
• Wire up IPC (onNewRequest, onDone)
• Update createInputPopupWindow() for dev/prod loading
• Verify @prompd/react CSS bundled in popup entry
• Electron builder: verify popup.html included in packaged app
• Test frameless window on Windows/macOS
• Remove old popup-input.html