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/client/client.go

@@ -75,7 +75,7 @@ func (c *Client) SendMessage(ctx context.Context, chatMessages []*store.Message,
 	// Map Messages
 	messages := slices.Map(chatMessages, func(m *store.Message) openai.ChatCompletionMessageParamUnion {
 		if m.Role == "user" {
-			return openai.UserMessage(m.Content)
+			return buildUserMessage(m)
 		}
 		return openai.AssistantMessage(m.Content)
 	})
@@ -292,3 +292,30 @@ func NewClient(baseURL *url.URL) *Client {
 	oaiClient := openai.NewClient(option.WithBaseURL(baseURL.String()))
 	return &Client{oaiClient: &oaiClient}
 }
+
+func buildUserMessage(m *store.Message) openai.ChatCompletionMessageParamUnion {
+	// Simple Text Message
+	if len(m.Images) == 0 {
+		return openai.UserMessage(m.Content)
+	}
+
+	// Build Multimodal Content Parts
+	parts := make([]openai.ChatCompletionContentPartUnionParam, 0, len(m.Images)+1)
+
+	// Add Image Parts
+	for _, imgURL := range m.Images {
+		parts = append(parts, openai.ImageContentPart(
+			openai.ChatCompletionContentPartImageImageURLParam{
+				URL: imgURL,
+			},
+		))
+	}
+
+	// Add Text Part
+	if m.Content != "" {
+		parts = append(parts, openai.TextContentPart(m.Content))
+	}
+
+	// Build User Message with Content Parts
+	return openai.UserMessage(parts)
+}

backend/internal/client/client_test.go

@@ -92,3 +92,35 @@ func TestSummarizeChat(t *testing.T) {
 		t.Logf("Output: %s", output)
 	}
 }
+
+func TestSendMessageWithImage(t *testing.T) {
+	t.Skip("requires live LLM API - run manually with: go test -run TestSendMessageWithImage ./internal/client/")
+
+	// Initialize Client
+	baseURL, err := url.Parse("https://llm-api.va.reichard.io/v1")
+	if err != nil {
+		t.Fatalf("Failed to parse base URL: %v", err)
+	}
+	client := NewClient(baseURL)
+
+	// Create Context
+	ctx, cancel := context.WithTimeout(context.Background(), 120*time.Second)
+	defer cancel()
+
+	// Generate Text Stream
+	_, err = client.SendMessage(ctx, []*store.Message{{
+		Role:    "user",
+		Content: "What is in this image?",
+		Images: []string{
+			"https://llm-api.va.reichard.io/v1/images/test.png",
+		},
+	}}, "vllm-qwen3-8b-vision", func(mc *MessageChunk) error {
+		if mc.Message != nil {
+			t.Logf("Received: %s", *mc.Message)
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatalf("Failed to generate text stream: %v", err)
+	}
+}