feat: Add image upload support for Spec Creation chat

Add the ability to attach images (JPEG, PNG) in the Spec Creation chat
interface for Claude to analyze during app specification creation.

Frontend changes:
- Add ImageAttachment interface to types.ts with id, filename, mimeType,
  base64Data, previewUrl, and size fields
- Update ChatMessage interface with optional attachments field
- Update useSpecChat hook to accept and send attachments via WebSocket
- Add file input, drag-drop support, and preview thumbnails to
  SpecCreationChat component with validation (5 MB max, JPEG/PNG only)
- Update ChatMessage component to render image attachments with
  click-to-enlarge functionality

Backend changes:
- Add ImageAttachment Pydantic schema with base64 validation
- Update spec_creation.py WebSocket handler to parse and validate
  image attachments from client messages
- Update spec_chat_session.py to format multimodal content blocks
  for Claude API using async generator pattern

Features:
- Drag-and-drop or click paperclip button to attach images
- Preview thumbnails with remove button before sending
- File type validation (image/jpeg, image/png)
- File size validation (5 MB maximum)
- Images display in chat history
- Click images to view full size
- Cross-platform compatible (Windows, macOS, Linux)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/hooks/useSpecChat.ts

@@ -3,7 +3,7 @@
  */
 
 import { useState, useCallback, useRef, useEffect } from 'react'
-import type { ChatMessage, SpecChatServerMessage, SpecQuestion } from '../lib/types'
+import type { ChatMessage, ImageAttachment, SpecChatServerMessage, SpecQuestion } from '../lib/types'
 
 type ConnectionStatus = 'disconnected' | 'connecting' | 'connected' | 'error'
 
@@ -21,7 +21,7 @@ interface UseSpecChatReturn {
   currentQuestions: SpecQuestion[] | null
   currentToolId: string | null
   start: () => void
-  sendMessage: (content: string) => void
+  sendMessage: (content: string, attachments?: ImageAttachment[]) => void
   sendAnswer: (answers: Record<string, string | string[]>) => void
   disconnect: () => void
 }
@@ -303,19 +303,20 @@ export function useSpecChat({
     setTimeout(checkAndSend, 100)
   }, [connect])
 
-  const sendMessage = useCallback((content: string) => {
+  const sendMessage = useCallback((content: string, attachments?: ImageAttachment[]) => {
     if (!wsRef.current || wsRef.current.readyState !== WebSocket.OPEN) {
       onError?.('Not connected')
       return
     }
 
-    // Add user message to chat
+    // Add user message to chat (with attachments for display)
     setMessages((prev) => [
       ...prev,
       {
         id: generateId(),
         role: 'user',
         content,
+        attachments,
         timestamp: new Date(),
       },
     ])
@@ -325,8 +326,23 @@ export function useSpecChat({
     setCurrentToolId(null)
     setIsLoading(true)
 
+    // Build message payload
+    const payload: { type: string; content: string; attachments?: Array<{ filename: string; mimeType: string; base64Data: string }> } = {
+      type: 'message',
+      content,
+    }
+
+    // Add attachments if present (send base64 data, not preview URL)
+    if (attachments && attachments.length > 0) {
+      payload.attachments = attachments.map((a) => ({
+        filename: a.filename,
+        mimeType: a.mimeType,
+        base64Data: a.base64Data,
+      }))
+    }
+
     // Send to server
-    wsRef.current.send(JSON.stringify({ type: 'message', content }))
+    wsRef.current.send(JSON.stringify(payload))
   }, [onError])
 
   const sendAnswer = useCallback((answers: Record<string, string | string[]>) => {