feat: Add authentication error handling to UI flow

Extend auth error detection to the web UI flow:

server/main.py:
- Fix setup_status() endpoint to check ~/.claude directory instead of
  non-existent .credentials.json file
- Add explanatory comments about Claude CLI credential storage changes

server/services/process_manager.py:
- Add AUTH_ERROR_PATTERNS for detecting auth errors in agent output
- Add is_auth_error() helper function
- Add AUTH_ERROR_HELP message template
- Update _stream_output() to detect auth errors in real-time
- Buffer last 20 lines to catch auth errors on process exit
- Broadcast clear help message to WebSocket clients when auth fails

start_ui.sh:
- Add Claude CLI installation check with helpful guidance
- Add ~/.claude directory check with login reminder
- Non-blocking warnings that don't prevent UI from starting

This ensures users get clear, actionable feedback when authentication
fails, whether using the CLI or the web UI.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

start_ui.sh

@@ -9,6 +9,27 @@ echo "  AutoCoder UI"
 echo "===================================="
 echo ""
 
+# Check if Claude CLI is installed
+if ! command -v claude &> /dev/null; then
+    echo "[!] Claude CLI not found"
+    echo ""
+    echo "    The agent requires Claude CLI to work."
+    echo "    Install it from: https://claude.ai/download"
+    echo ""
+    echo "    After installing, run: claude login"
+    echo ""
+else
+    echo "[OK] Claude CLI found"
+    # Note: Claude CLI no longer stores credentials in ~/.claude/.credentials.json
+    # We can't reliably check auth status without making an API call
+    if [ -d "$HOME/.claude" ]; then
+        echo "     (If you're not logged in, run: claude login)"
+    else
+        echo "[!] Claude CLI not configured - run 'claude login' first"
+    fi
+fi
+echo ""
+
 # Check if Python is available
 if ! command -v python3 &> /dev/null; then
     if ! command -v python &> /dev/null; then