fix: prevent IndexError in focus_index with invalid input

[projectakshith]

this PR fixes a crash encountered during the installation process when interacting with the TUI menus (specifically in the disk configuration).

the bug:
the focus_index method in menu_item.py currently attempts to access the enabled list using a raw index provided by the key press logic. if a user presses a number key (e.g '3') that corresponds to an index larger than the current list size (e.g. in a menu with only 2 items), it raises an IndexError and crashes the installer.

the fix:
i have added a guard clause to focus_index that checks if the requested index is within valid bounds (0 <= index < len(enabled)). this matches the safety behavior already present in focus_first and focus_last.

def focus_index(self, index: int) -> None:
        enabled = self.get_enabled_items()

        # fix: this prevents crashing on invalid key presses
        if not (0 <= index < len(enabled)):
            return

        self.focus_item = enabled[index]

verification:
i reproduced this crash on the latest ISO. i manually patched the file in /usr/lib/python3.13/site-packages/archinstall/tui/menu_item.py with this fix, verified that the crash no longer occurs, and successfully completed an installation.

Traceback

[2026-01-30 13:28:29] - ERROR - Traceback (most recent call last):
...
  File "/usr/lib/python3.13/site-packages/archinstall/tui/curses_menu.py", line 1216, in _process_input_key
    self._item_group.focus_index(key - 49)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/usr/lib/python3.13/site-packages/archinstall/tui/menu_item.py", line 284, in focus_index
    self.focus_item = enabled[index]
                      ~~~~~~~^^^^^^^
IndexError: list index out of range

[svartkanin]

The menu has been rewritten to use textual and this is no longer used on master.
With this I realized that jumping to a given entry with 1-9 does not work anymore. I wonder if it's a much used feature or we can drop it 🤔

[projectakshith]

i think it is worth keeping, i mean having a secondary method to navigate will always be better for accessibility.

if for some reason the arrow keys aren't convenient, or they get mangled in web consoles or certain remote setups (where escape codes don't pass through correctly), number keys could be a reliable fallback that the users could still use imo