fix: remove dead rollback branch in applyGlobalHotKey() (M2)

[sfegette]

Summary

In AppDelegate.applyGlobalHotKey(), the catch block contains:

if currentHotKey != previousHotKey { ... }

This condition is never true because activateGlobalHotKey only mutates currentHotKey after a successful registration. The "grapfel lost its global shortcut" message path is unreachable.

Fix

• Remove the dead if branch
• Add a short comment in the catch block explaining the invariant: the old hotkey remains active on failure because currentHotKey is only updated on success

Notes

Low-risk cleanup — behavior is correct, this is just dead code removal.