feat(chat): add optional photo upload support

Add vision/multimodal support to chat, allowing users to send images
alongside or instead of text prompts. Images are transmitted and persisted
as base64 data URLs.

Backend:
- Add Images []string to Message struct for persistence
- Add Images []string to GenerateTextRequest with relaxed validation
- Build multimodal user messages using OpenAI SDK content parts
- Pass images through from handlers to client
- Deep-copy Images slice in message cloning

Frontend:
- Add images?: string[] to Message and GenerateTextRequest types
- Add image selection state and file input handler
- Add camera icon button, hidden file input, and image preview strip
- Render images in user message bubbles
- Pass images through to GenerateTextRequest

Tests:
- Add TestSendMessageWithImage for vision model testing

backend/internal/store/types.go

@@ -43,6 +43,7 @@ type Message struct {
 	Role     string              `json:"role"`
 	Thinking string              `json:"thinking"`
 	Content  string              `json:"content"`
+	Images   []string            `json:"images,omitempty"`
 	Status   MessageStatus       `json:"status,omitempty"`
 	Stats    *types.MessageStats `json:"stats,omitempty"`
 }