feat: Story 11 - Left-align text and add syntax highlighting

Implemented Story 11: Text Alignment and Syntax Highlighting
- Removed center-alignment from chat container (text-align: center)
- Added left-alignment to markdown body and code blocks
- Integrated react-syntax-highlighter with react-markdown
- Added syntax highlighting for code blocks using oneDark theme
- Supports JavaScript, TypeScript, Rust, Python, JSON, Markdown, Shell, and more
- Inline code maintains simple styling without full highlighting
- All code blocks are now left-aligned for better readability

Fixed: Agent over-aggressive file writing behavior
- Refined system prompt to distinguish between:
  * 'show/example/how does' → respond with code in chat
  * 'create/add/implement/fix' → use write_file tool
- Removed aggressive AGENT DIRECTIVE prefix from user messages
- Softened reminder message to reflect nuanced behavior
- Agent can now both teach (show examples) and implement (write files)

Updated Specs
- Added Text Alignment and Readability section to UI_UX.md
- Added Syntax Highlighting section with implementation details
- Updated SDSW workflow: acceptance criteria marked complete only after user acceptance

Dependencies
- Added react-syntax-highlighter and @types/react-syntax-highlighter

src-tauri/src/commands/chat.rs

@@ -53,16 +53,6 @@ pub async fn chat(
     // 3. Agent Loop
     let mut current_history = messages.clone();
 
-    // Prefix user messages with reminder for stubborn models
-    for msg in &mut current_history {
-        if msg.role == Role::User && !msg.content.starts_with("[AGENT DIRECTIVE]") {
-            msg.content = format!(
-                "[AGENT DIRECTIVE: You must use write_file tool to implement changes. Never suggest code.]\n\n{}",
-                msg.content
-            );
-        }
-    }
-
     // Inject System Prompt
     current_history.insert(
         0,
@@ -74,12 +64,12 @@ pub async fn chat(
         },
     );
 
-    // Inject aggressive reminder as a second system message
+    // Inject reminder as a second system message
     current_history.insert(
         1,
         Message {
             role: Role::System,
-            content: "CRITICAL REMINDER: When the user asks you to create, modify, or implement code, you MUST call the write_file tool with the complete file content. DO NOT output code in markdown blocks. DO NOT suggest what the user should do. TAKE ACTION IMMEDIATELY using tools.".to_string(),
+            content: "REMINDER: Distinguish between showing examples (use code blocks in chat) vs implementing changes (use write_file tool). Keywords like 'show me', 'example', 'how does' = chat response. Keywords like 'create', 'add', 'implement', 'fix' = use tools.".to_string(),
             tool_calls: None,
             tool_call_id: None,
         },