feat(settings): add owned string allocation for Settings struct

- Add `toOwned` method to Settings that creates heap-allocated copies
  of all string fields
- Update load function to use heap-owned string allocations by
  default when creating new settings files
- Update error handling to ensure all string fields are heap-owned
  when parsing fails and defaults are used
- Add documentation for the new toOwned method explaining memory
  ownership requirements

Author: yizhinailong

src/settings.zig

@@ -51,13 +51,29 @@ pub const Settings = struct {
         proxy: []const u8 = "",
     };
 
+    /// Return a copy of this Settings with all string fields heap-allocated.
+    /// The caller owns the returned value and must free all string fields.
+    fn toOwned(self: Settings, allocator: std.mem.Allocator) !Settings {
+        return Settings{
+            .version_map_url = try allocator.dupe(u8, self.version_map_url),
+            .zls_vmu = try allocator.dupe(u8, self.zls_vmu),
+            .mirror_list_url = try allocator.dupe(u8, self.mirror_list_url),
+            .use_color = self.use_color,
+            .always_force_install = self.always_force_install,
+            .preferred_mirror = try allocator.dupe(u8, self.preferred_mirror),
+            .mirror_updated_at = self.mirror_updated_at,
+            .proxy = try allocator.dupe(u8, self.proxy),
+            .path = self.path,
+        };
+    }
+
     /// Load settings from a JSON file, or create with defaults if not found.
     /// Takes ownership of the `path` parameter.
     pub fn load(allocator: std.mem.Allocator, io: std.Io, path: []const u8) !Settings {
         const file = std.Io.Dir.cwd().openFile(io, path, .{}) catch |err| switch (err) {
             error.FileNotFound => {
-                // Create new settings file with defaults
-                var settings = default;
+                // Create new settings file with defaults; all strings must be heap-owned.
+                var settings = try default.toOwned(allocator);
                 settings.path = path;
                 try settings.save(allocator, io);
                 return settings;
@@ -77,8 +93,8 @@ pub const Settings = struct {
             content,
             .{ .ignore_unknown_fields = true },
         ) catch {
-            // If parsing fails, return defaults
-            var settings = default;
+            // If parsing fails, return defaults; all strings must be heap-owned.
+            var settings = try default.toOwned(allocator);
             settings.path = path;
             return settings;
         };