feat: implement Tab/Shift+Tab navigation and remap autocomplete

[cap153]

Description

This PR implements standard keyboard navigation using Tab and Shift+Tab to move selection up and down in the launcher list, addressing a common user expectation for GUI launchers.

Additionally, the autocomplete functionality (previously bound to Tab) has been remapped to the Right Arrow key to prevent conflict and align with shell/terminal behavior.

Changes

1. Navigation Logic:

   • Tab: Now triggers FocusNext (Select next item).
   • Shift+Tab: Now triggers FocusPrevious (Select previous item).
   • Right Arrow: Now triggers Autocomplete (formerly TabPress).

2. Refactor:

   • Renamed Message::TabPress to Message::Autocomplete. Since the action is no longer bound to the Tab key, this name better reflects the semantic intent.

3. Focus Handling:

   • Introduced Message::KeepFocus.
   • Bound text_input's .on_tab() to Message::KeepFocus.
   • Rationale: The text_input widget captures the Tab key by default. By binding it to a no-op message (KeepFocus), we prevent the widget from consuming the event logic while keeping the input focused, allowing the global subscription to correctly handle Tab and Shift+Tab events for navigation.

Testing

Tested on Arch Linux by replacing the system binary:

• Tab: Correctly moves selection down.
• Shift+Tab: Correctly moves selection up.
• Right Arrow: Correctly autocompletes the search query (e.g., completing a path).
• Typing: Focus remains in the search bar during navigation.

Related Issue

Closes #312