fix: resolve "sorry something went wrong" errors and show images in chat

Three fixes from beta tester session feedback:

1. MCP error handling (backend/app/services/assistant_chat_service.py)
   - The MCP Microsoft Learn integration was catching only BadRequestError.
     Any other error type (APIStatusError, APIConnectionError, timeout) from
     the external MCP server propagated as a 502, causing the generic error.
   - Now catches all Exception types when MCP is active and retries without
     MCP using the stable client.messages.create endpoint.

2. Frontend error UX (frontend/src/pages/AssistantChatPage.tsx)
   - catch {} was silently swallowing all errors and inserting a generic
     assistant message. Now: differentiates 429 (rate limit) vs 502/503
     (AI unavailable), removes the optimistic user message on failure,
     restores the failed message to the input so users can retry without
     retyping, and logs errors to console for debugging.

3. Image attachments visible in chat (frontend/src/components/assistant/ChatMessage.tsx)
   - Uploaded images were sent to the AI correctly but never shown in the
     chat thread. Now captures preview URLs before clearing pendingUploads
     and renders thumbnails above the user bubble, clickable to full size.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/assistant/ChatMessage.tsx

@@ -7,9 +7,10 @@ interface ChatMessageProps {
   role: 'user' | 'assistant'
   content: string
   suggestedFlows?: SuggestedFlow[]
+  imageUrls?: string[]
 }
 
-export function ChatMessage({ role, content, suggestedFlows }: ChatMessageProps) {
+export function ChatMessage({ role, content, suggestedFlows, imageUrls }: ChatMessageProps) {
   return (
     <div className={`flex gap-3 ${role === 'user' ? 'flex-row-reverse' : ''}`}>
       {/* Avatar */}
@@ -25,6 +26,21 @@ export function ChatMessage({ role, content, suggestedFlows }: ChatMessageProps)
 
       {/* Content */}
       <div className={`max-w-[80%] space-y-2 ${role === 'user' ? 'text-right' : ''}`}>
+        {/* Image attachments (user messages only) */}
+        {role === 'user' && imageUrls && imageUrls.length > 0 && (
+          <div className={`flex flex-wrap gap-2 ${role === 'user' ? 'justify-end' : ''}`}>
+            {imageUrls.map((url, i) => (
+              <a key={i} href={url} target="_blank" rel="noopener noreferrer">
+                <img
+                  src={url}
+                  alt={`Attachment ${i + 1}`}
+                  className="h-24 w-auto max-w-[200px] rounded-xl object-cover border border-border cursor-pointer hover:opacity-90 transition-opacity"
+                />
+              </a>
+            ))}
+          </div>
+        )}
+
         <div
           className={`rounded-2xl px-4 py-3 text-[0.875rem] leading-relaxed ${
             role === 'user'