Add HF_ENDPOINT in the app settings (#1783)

## Motivation
Some users (primarily in China) are unable to access HuggingFace.co. HF
supports the HF_ENDPOINT env variable, and we also support it, but there
is no way to easily do that from the app currently.

## Test Plan

### Manual Testing

hf-mirror works
empty field works
google.com endpoint fails

app/EXO/EXO/ExoProcessController.swift

@@ -4,6 +4,7 @@ import Foundation
 
 private let customNamespaceKey = "EXOCustomNamespace"
 private let hfTokenKey = "EXOHFToken"
+private let hfEndpointKey = "EXOHFEndpoint"
 private let enableImageModelsKey = "EXOEnableImageModels"
 private let offlineModeKey = "EXOOfflineMode"
 private let onboardingCompletedKey = "EXOOnboardingCompleted"
@@ -53,6 +54,14 @@ final class ExoProcessController: ObservableObject {
             UserDefaults.standard.set(hfToken, forKey: hfTokenKey)
         }
     }
+    @Published var hfEndpoint: String = {
+        return UserDefaults.standard.string(forKey: hfEndpointKey) ?? ""
+    }()
+    {
+        didSet {
+            UserDefaults.standard.set(hfEndpoint, forKey: hfEndpointKey)
+        }
+    }
     @Published var enableImageModels: Bool = {
         return UserDefaults.standard.bool(forKey: enableImageModelsKey)
     }()
@@ -273,6 +282,9 @@ final class ExoProcessController: ObservableObject {
         if !hfToken.isEmpty {
             environment["HF_TOKEN"] = hfToken
         }
+        if !hfEndpoint.isEmpty {
+            environment["HF_ENDPOINT"] = hfEndpoint
+        }
         if enableImageModels {
             environment["EXO_ENABLE_IMAGE_MODELS"] = "true"
         }

app/EXO/EXO/Views/SettingsView.swift

@@ -12,6 +12,7 @@ struct SettingsView: View {
 
     @State private var pendingNamespace: String = ""
     @State private var pendingHFToken: String = ""
+    @State private var pendingHFEndpoint: String = ""
     @State private var pendingEnableImageModels = false
     @State private var pendingOfflineMode = false
     @State private var needsRestart = false
@@ -42,6 +43,7 @@ struct SettingsView: View {
         .onAppear {
             pendingNamespace = controller.customNamespace
             pendingHFToken = controller.hfToken
+            pendingHFEndpoint = controller.hfEndpoint
             pendingEnableImageModels = controller.enableImageModels
             pendingOfflineMode = controller.offlineMode
             needsRestart = false
@@ -74,6 +76,17 @@ struct SettingsView: View {
                     .foregroundColor(.secondary)
             }
 
+            Section {
+                LabeledContent("HuggingFace Endpoint") {
+                    TextField("default", text: $pendingHFEndpoint)
+                        .textFieldStyle(.roundedBorder)
+                        .frame(width: 200)
+                }
+                Text("Defaults to huggingface.co. Use a mirror (e.g. hf-mirror.com) for China.")
+                    .font(.caption)
+                    .foregroundColor(.secondary)
+            }
+
             Section {
                 Toggle("Offline Mode", isOn: $pendingOfflineMode)
                 Text("Skip internet checks and use only locally available models.")
@@ -454,6 +467,7 @@ struct SettingsView: View {
 
     private var hasGeneralChanges: Bool {
         pendingNamespace != controller.customNamespace || pendingHFToken != controller.hfToken
+            || pendingHFEndpoint != controller.hfEndpoint
             || pendingOfflineMode != controller.offlineMode
     }
 
@@ -464,6 +478,7 @@ struct SettingsView: View {
     private func applyGeneralSettings() {
         controller.customNamespace = pendingNamespace
         controller.hfToken = pendingHFToken
+        controller.hfEndpoint = pendingHFEndpoint
         controller.offlineMode = pendingOfflineMode
         restartIfRunning()
     }

justfile

@@ -31,6 +31,10 @@ build-dashboard:
 package:
     uv run pyinstaller packaging/pyinstaller/exo.spec
 
+build-app: package
+    xcodebuild build -project app/EXO/EXO.xcodeproj -scheme EXO -configuration Debug -derivedDataPath app/EXO/build
+    @echo "\nBuild complete. Run with:\n  open {{justfile_directory()}}/app/EXO/build/Build/Products/Debug/EXO.app"
+
 clean:
     rm -rf **/__pycache__
     rm -rf target/