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_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)
+	}
+}