Add timer to temporarily lock extension settings (issue #92)

[AndyPLsql]

This PR implements a simple timer that lets users temporarily lock the extension configuration.

What’s new

• Added a Lock button next to the existing settings.
• Added a numeric input for lock duration in hours.
• The field accepts decimal values in hours

How it works

• When the user enters a value and clicks Lock:
  • The “Enabled” control is disabled.
  • The “Sites to block” settings are disabled.
  • The Lock button itself becomes disabled (so the lock cannot be immediately changed).
• While the timer is active, the user cannot modify these settings.
• When the timer expires:
  • All locked controls become enabled again.
  • The user can change “Enabled” and “Sites to block” as usual.