Improve bin/dev kill error handling for process termination

Enhance the terminate_processes method to distinguish between ESRCH
(process already stopped) and EPERM (permission denied) errors, providing
clearer feedback to users when process termination fails.

Key improvements:
- Separate handling for ESRCH vs EPERM exceptions
- User warning when permission is denied for a process
- Clearer indication of what actually happened during process termination

This change aligns with the pattern implemented in react_on_rails-demos
PR #42 for better process management.

Fixes #1858

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

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

Author: justin808

lib/react_on_rails/dev/server_manager.rb

@@ -70,8 +70,12 @@ def find_process_pids(pattern)
         def terminate_processes(pids)
           pids.each do |pid|
             Process.kill("TERM", pid)
-          rescue StandardError
+          rescue Errno::ESRCH
+            # Process already stopped - this is fine
             nil
+          rescue Errno::EPERM
+            # Permission denied - warn the user
+            puts "   ⚠️  Process #{pid} - permission denied (process owned by another user)"
           end
         end