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/api/generation.go

@@ -2,6 +2,7 @@ package api
 
 import (
 	"errors"
+	"slices"
 	"sync"
 
 	"github.com/google/uuid"
@@ -153,6 +154,7 @@ func cloneStoreMessage(msg *store.Message) *store.Message {
 
 	// Clone Message
 	cloned := *msg
+	cloned.Images = slices.Clone(msg.Images)
 	if msg.Stats != nil {
 		stats := *msg.Stats
 		cloned.Stats = &stats