macos: implement audio bell support with bell-audio-path

Extends the macOS bell implementation to support the `audio` bell
feature by playing a user-specified audio file via NSSound.

Previously, macOS only supported the `system` feature (NSSound.beep()).
This change adds support for:
- `audio` bell feature: plays the file at `bell-audio-path` using
  NSSound, respecting the `bell-audio-volume` setting
- Adds `cval()` to the `Path` type so it can be returned via the C API

Also removes the "(GTK only)" restriction from `bell-audio-path` and
`bell-audio-volume` documentation, as these options now work on macOS.

Example config:
  bell-features = audio
  bell-audio-path = /System/Library/Sounds/Glass.aiff
  bell-audio-volume = 0.8

src/config/Config.zig

@@ -3087,7 +3087,7 @@ keybind: Keybinds = .{},
 /// the path is not absolute, it is considered relative to the directory of the
 /// configuration file that it is referenced from, or from the current working
 /// directory if this is used as a CLI flag. The path may be prefixed with `~/`
-/// to reference the user's home directory. (GTK only)
+/// to reference the user's home directory.
 ///
 /// Available since: 1.2.0
 @"bell-audio-path": ?Path = null,
@@ -3095,7 +3095,6 @@ keybind: Keybinds = .{},
 /// If `audio` is an enabled bell feature, this is the volume to play the audio
 /// file at (relative to the system volume). This is a floating point number
 /// ranging from 0.0 (silence) to 1.0 (as loud as possible). The default is 0.5.
-/// (GTK only)
 ///
 /// Available since: 1.2.0
 @"bell-audio-volume": f64 = 0.5,

src/config/path.zig

@@ -32,6 +32,14 @@ pub const Path = union(enum) {
         return std.meta.eql(self, other);
     }
 
+    /// Returns the path as a C-compatible null-terminated string pointer.
+    pub fn cval(self: Path) [*:0]const u8 {
+        return switch (self) {
+            .optional => |path| path.ptr,
+            .required => |path| path.ptr,
+        };
+    }
+
     /// Parse the input and return a Path. A leading `?` indicates that the path
     /// is _optional_ and an error should not be logged or displayed to the user
     /// if that path does not exist. Otherwise the path is required and an error