Treat Swift/Xcode build warnings as errors (#1322)

## Motivation

Warnings that go unchecked tend to accumulate and hide real issues.
Treating them as errors ensures they are addressed immediately, both
locally during development and in CI.

## Changes

Added `SWIFT_TREAT_WARNINGS_AS_ERRORS = YES` and
`GCC_TREAT_WARNINGS_AS_ERRORS = YES` to the **project-level** Debug and
Release build configurations in `project.pbxproj`. This applies to all
targets (EXO, EXOTests, EXOUITests).

## Why It Works

Xcode's `SWIFT_TREAT_WARNINGS_AS_ERRORS` and
`GCC_TREAT_WARNINGS_AS_ERRORS` build settings promote Swift and C/ObjC
warnings to errors at compile time. Setting them at the project level
means all targets inherit the policy without needing per-target or
CI-level overrides.

## Test Plan

### Manual Testing
- Built the EXO scheme in Release configuration with `xcodebuild` — no
warning-as-error failures from Swift or C/ObjC sources.

### Automated Testing
- CI already builds with `-configuration Release`, so it will
automatically enforce warnings-as-errors via the inherited project
settings — no CI changes needed.

app/EXO/EXO.xcodeproj/project.pbxproj

@@ -342,6 +342,8 @@
 				SDKROOT = macosx;
 				SWIFT_ACTIVE_COMPILATION_CONDITIONS = "DEBUG $(inherited)";
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
+				SWIFT_TREAT_WARNINGS_AS_ERRORS = YES;
+				GCC_TREAT_WARNINGS_AS_ERRORS = YES;
 			};
 			name = Debug;
 		};
@@ -397,6 +399,8 @@
 				MTL_FAST_MATH = YES;
 				SDKROOT = macosx;
 				SWIFT_COMPILATION_MODE = wholemodule;
+				SWIFT_TREAT_WARNINGS_AS_ERRORS = YES;
+				GCC_TREAT_WARNINGS_AS_ERRORS = YES;
 			};
 			name = Release;
 		};