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

frontend/src/components/chatManager.ts

@@ -37,6 +37,7 @@ Alpine.data('chatManager', () => ({
 
   selectedModel: '',
   inputMessage: '',
+  selectedImages: [] as string[],
   error: '',
 
   selectedChatID: null as string | null,
@@ -88,10 +89,12 @@ Alpine.data('chatManager', () => ({
 
   async sendMessage() {
     const message = this.inputMessage.trim();
-    if (!message || this.loading) return;
+    if ((!message && this.selectedImages.length === 0) || this.loading) return;
 
     // Update State
+    const images = [...this.selectedImages];
     this.inputMessage = '';
+    this.selectedImages = [];
     this.loading = true;
     this.error = '';
 
@@ -121,6 +124,7 @@ Alpine.data('chatManager', () => ({
       role: 'user',
       thinking: '',
       content: message,
+      images: images,
       created_at: new Date().toISOString(),
     });
     currentChat.message_count += 1;
@@ -128,7 +132,7 @@ Alpine.data('chatManager', () => ({
     try {
       await sendMessage(
         this.selectedChatID === IN_PROGRESS_UUID ? '' : this.selectedChatID,
-        { model: this.selectedModel, prompt: message },
+        { model: this.selectedModel, prompt: message, images },
         (chunk: MessageChunk) => {
           if (chunk.chat) this.activeStreamChatID = chunk.chat.id;
           this.applyMessageChunk(chunk);

frontend/src/types/index.ts

@@ -16,6 +16,7 @@ export interface Message {
   role: 'user' | 'assistant';
   thinking: string;
   content: string;
+  images?: string[];
   status?: MessageStatus;
   stats?: MessageStats;
 }
@@ -69,6 +70,7 @@ export interface GenerateImageRequest {
 export interface GenerateTextRequest {
   model: string;
   prompt: string;
+  images?: string[];
 }
 
 export interface ChatListResponse {