opt: display conflicting key combination in keybinding warning

[shoobham84]

Summary

When a keybinding conflict is detected during config loading, the warning only prints file paths and line numbers. This makes it hard to see which key combinations caused the collision.

Wire up existing mod_to_string() helper and resolve keysym name so key combination is visible in warning output.

Before and After Comparison

Before:

[WARNING] Key binding conflict in keymode default:
  File "/tmp/test_conflict.conf", line 1
  File "/tmp/test_conflict.conf", line 2

After:

[WARNING] Key binding conflict for Super + r in keymode default:
  File "/tmp/test_conflict.conf", line 1
  File "/tmp/test_conflict.conf", line 2