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)
+}