Replaced the local `termbox2.h` header file with a proper dependency managed by the Zig package manager. This improves maintainability and makes it easier to track and update the library in the future.
The previous `termbox2.h` contained a custom `tb_get_cell` function that is not present in the upstream repository. This function has been re-implemented in Zig (`src/tui/termbox_extras.zig`) to maintain compatibility, especially for the failed-login "cascade" animation.
This change also involved updating `build.zig` and `build.zig.zon` to use the new dependency.
João Lucas