refactor: Use zig fetch for termbox2 dependency

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.

build.zig

@@ -68,13 +68,17 @@ pub fn build(b: *std.Build) !void {
     const clap = b.dependency("clap", .{ .target = target, .optimize = optimize });
     exe.root_module.addImport("clap", clap.module("clap"));
 
-    exe.addIncludePath(b.path("include"));
+    const termbox_dep = b.dependency("termbox2", .{
+        .target = target,
+        .optimize = optimize,
+    });
+
     exe.linkSystemLibrary("pam");
     if (enable_x11_support) exe.linkSystemLibrary("xcb");
     exe.linkLibC();
 
     const translate_c = b.addTranslateC(.{
-        .root_source_file = b.path("include/termbox2.h"),
+        .root_source_file = termbox_dep.path("termbox2.h"),
         .target = target,
         .optimize = optimize,
     });

build.zig.zon

@@ -12,6 +12,10 @@
             .url = "https://github.com/Kawaii-Ash/zigini/archive/2ed3d417f17fab5b0ee8cad8a63c6d62d7ac1042.tar.gz",
             .hash = "zigini-0.3.1-BSkB7XJGAAB2E-sKyzhTaQCBlYBL8yqzE4E_jmSY99sC",
         },
+        .termbox2 = .{
+            .url = "git+https://github.com/termbox/termbox2#8ee9dc17e1ca61c630f91db0aa7f81fa29a32040",
+            .hash = "N-V-__8AAKvjBAAUF2KVdkHsNs7L5EEZYzBnrJTBvj-baBMZ",
+        },
     },
     .paths = .{""},
 }

src/tui/TerminalBuffer.zig

@@ -2,6 +2,7 @@ const std = @import("std");
 const builtin = @import("builtin");
 const interop = @import("../interop.zig");
 const Cell = @import("Cell.zig");
+const termbox_extras = @import("termbox_extras.zig");
 
 const Random = std.Random;
 
@@ -113,8 +114,8 @@ pub fn cascade(self: TerminalBuffer) bool {
             var cell: termbox.tb_cell = undefined;
             var cell_under: termbox.tb_cell = undefined;
 
-            _ = termbox.tb_get_cell(@intCast(x), @intCast(y - 1), 1, &cell);
+            _ = termbox_extras.tb_get_cell(@intCast(x), @intCast(y - 1), 1, &cell);
-            _ = termbox.tb_get_cell(@intCast(x), @intCast(y), 1, &cell_under);
+            _ = termbox_extras.tb_get_cell(@intCast(x), @intCast(y), 1, &cell_under);
 
             const char: u8 = @truncate(cell.ch);
             if (std.ascii.isWhitespace(char)) continue;

src/tui/termbox_extras.zig

@@ -0,0 +1,26 @@
+const std = @import("std");
+const interop = @import("../interop.zig");
+const termbox = interop.termbox;
+
+pub fn tb_get_cell(x: c_int, y: c_int, back: c_int, cell: *termbox.tb_cell) c_int {
+    if (back == 0) {
+        return termbox.TB_ERR;
+    }
+
+    const width = termbox.tb_width();
+    const height = termbox.tb_height();
+
+    if (x < 0 or x >= width or y < 0 or y >= height) {
+        return termbox.TB_ERR_OUT_OF_BOUNDS;
+    }
+
+    const buffer = termbox.tb_cell_buffer();
+    if (buffer == null) {
+        return termbox.TB_ERR_NOT_INIT;
+    }
+
+    const index = y * width + x;
+    cell.* = buffer[@intCast(index)];
+
+    return termbox.TB_OK;
+}