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>

backend/app/services/assistant_chat_service.py

@@ -303,6 +303,8 @@ async def _call_anthropic_cached(
             }
         ]
 
+    _mcp_active = mcp_servers is not anthropic.NOT_GIVEN
+
     try:
         response = await client.beta.messages.create(
             model=settings.AI_MODEL_ANTHROPIC,
@@ -313,12 +315,22 @@ async def _call_anthropic_cached(
             tools=tools,
             betas=["mcp-client-2025-11-20"],
         )
-    except anthropic.BadRequestError as e:
-        # MCP server failures (rate limits, connection errors) should not
-        # block the assistant entirely — retry without MCP tools.
-        if "MCP server" in str(e) and mcp_servers is not anthropic.NOT_GIVEN:
-            logger.warning("MCP server error, retrying without MCP: %s", e)
-            response = await client.beta.messages.create(
+    except Exception as e:
+        # MCP server failures surface as many error types — BadRequestError,
+        # APIStatusError, APIConnectionError, APITimeoutError.  Always retry
+        # without MCP when MCP was active, so a flaky external server never
+        # blocks the assistant entirely.
+        _is_mcp_error = _mcp_active and (
+            "MCP server" in str(e)
+            or "mcp" in type(e).__name__.lower()
+            or isinstance(e, (anthropic.BadRequestError, anthropic.APIStatusError))
+        )
+        if _is_mcp_error:
+            logger.warning(
+                "MCP server error (%s), retrying without MCP: %s",
+                type(e).__name__, e,
+            )
+            response = await client.messages.create(
                 model=settings.AI_MODEL_ANTHROPIC,
                 max_tokens=max_tokens,
                 system=system_blocks,