Document custom.txt default-settings mechanism in BUILD.md

BUILD.md

@@ -114,6 +114,35 @@ Dart 3.12 rejects `const Map<FontWeight, String>{...}`. Change `const` to `final
 be wiped by `flutter pub cache clean`.** To make it permanent, copy the patched file into
 `./local_google_fonts/` and use `dependency_overrides: google_fonts: { path: ... }`.
 
+### 2.6. `flutter/macos/Runner/custom.txt` (default settings via Rust's `load_custom_client`)
+
+RustDesk's Rust core reads `Contents/Resources/custom.txt` on startup
+(see `src/common.rs::load_custom_client` + `read_custom_client`). It accepts two
+keys: `default-settings` (fills `DEFAULT_SETTINGS`; user config still wins) and
+`override-settings` (fills `OVERWRITE_SETTINGS`; user can't override).
+
+**Current contents:**
+```json
+{
+  "app-name": "RustDesk",
+  "default-settings": {
+    "enable-audio": "N"
+  }
+}
+```
+
+Effect: the "Enable audio" option in Settings is **off by default** for new
+installs. Users can still flip it on. To add more defaults, follow the
+`KEYS_SETTINGS` whitelist in `libs/hbb_common/src/config.rs:3130`.
+
+**Two parts to make this work:**
+
+1. The file `flutter/macos/Runner/custom.txt` must exist (it does in git).
+2. `flutter/macos/Runner.xcodeproj/project.pbxproj` must list `custom.txt` in
+   the `PBXResourcesBuildPhase`'s `files` list. We added it under UUIDs
+   `AABBCC000100000000000001` (BuildFile) and `AABBCC000200000000000002`
+   (FileReference). If you regenerate the Xcode project, re-add it.
+
 ---
 
 ## 3. Build steps (from a clean checkout)