fix(config): never let app.json I/O stop startup and reject numeric enum values

YKanBan.Tests/Storage/AppData/AppConfigStoreTests.cs

@@ -53,6 +53,39 @@ public class AppConfigStoreTests {
         Assert.AreEqual(AppConfig.CurrentFormatVersion, store.Load().Version);
     }
 
+    [TestMethod]
+    public void LoadNumericEnumValueIsRejectedAndBackedUp() {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+
+        // Integer enum values are not accepted; the file is treated like corrupt.
+        File.WriteAllText(path, """{ "theme": 1 }""");
+        var store = new AppConfigStore(path);
+
+        AppConfig config = store.Load();
+
+        Assert.AreEqual(ThemeOption.FollowSystem, config.Theme);
+        Assert.IsTrue(File.Exists(path + ".bak"));
+    }
+
+    [TestMethod]
+    public void LoadUnreadableFileReturnsDefaults() {
+        // File sharing is only reliably enforced on Windows, so this branch is exercised there.
+        if (!OperatingSystem.IsWindows()) {
+            return;
+        }
+
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        File.WriteAllText(path, """{ "version": 1 }""");
+
+        // Hold the file exclusively so the store's read fails with an I/O error.
+        using FileStream hold = File.Open(path, FileMode.Open, FileAccess.Read, FileShare.None);
+        var store = new AppConfigStore(path);
+
+        Assert.AreEqual(AppConfig.CurrentFormatVersion, store.Load().Version);
+    }
+
     [TestMethod]
     public void LoadReadsKebabCaseKeysAndEnumValues() {
         using var directory = new TempDirectory();

YKanBan/Storage/AppData/AppConfigStore.cs

@@ -4,18 +4,23 @@ using Newtonsoft.Json.Converters;
 namespace YKanBan.Storage.AppData;
 
 /// <summary>
-/// Loads and saves app.json with graceful degradation: a missing file yields
-/// defaults; a corrupt file is backed up to app.json.bak and then defaults are
-/// used. Saving writes the file directly (no atomic replace) — losing the last
-/// session's settings to a crash is an accepted trade-off.
+/// Loads and saves app.json with graceful degradation: a missing or unreadable
+/// file yields defaults; a corrupt file is backed up to app.json.bak and then
+/// defaults are used. Saving writes the file directly (no atomic replace) —
+/// losing the last session's settings to a crash is an accepted trade-off.
 /// </summary>
 public sealed class AppConfigStore {
     private static readonly JsonSerializerSettings SerializerSettings = new() {
         Formatting = Formatting.Indented,
         NullValueHandling = NullValueHandling.Ignore,
 
-        // Enums round-trip through the kebab-case strings declared via [EnumMember].
-        Converters = { new StringEnumConverter() },
+        // Enums round-trip through the kebab-case strings declared via [EnumMember];
+        // numeric enum values are rejected like any other unknown value.
+        // YYC MARK: non-enum fields (bool/int/string) are still coerced by
+        // Newtonsoft (for example 0 or "true" to bool, "500" or 500.0 to int);
+        // a wrongly typed non-enum value is accepted and converted rather than
+        // treated as invalid. That leniency is accepted here.
+        Converters = { new StringEnumConverter { AllowIntegerValues = false } },
     };
 
     /// <summary>
@@ -31,17 +36,24 @@ public sealed class AppConfigStore {
 
     /// <summary>
     /// Loads the configuration. A missing file yields defaults; a corrupt file
-    /// is backed up to app.json.bak and then defaults are used.
+    /// is backed up to app.json.bak and then defaults are used; a file that
+    /// exists but cannot be read (I/O or permission error) also yields defaults.
     /// </summary>
     /// <returns>The loaded configuration, or defaults when none could be read.</returns>
-    /// <exception cref="IOException">The file exists but cannot be read.</exception>
     public AppConfig Load() {
         // Missing file is a normal first-run state: run with defaults.
         if (!File.Exists(FilePath)) {
             return new AppConfig();
         }
 
-        string json = File.ReadAllText(FilePath);
+        string json;
+        try {
+            json = File.ReadAllText(FilePath);
+        } catch (Exception exception) when (exception is IOException or UnauthorizedAccessException) {
+            // A bad app.json must not stop startup.
+            return new AppConfig();
+        }
+
         try {
             // A JSON null literal deserializes to null; treat it like defaults.
             AppConfig? config = JsonConvert.DeserializeObject<AppConfig>(json, SerializerSettings);
@@ -54,8 +66,12 @@ public sealed class AppConfigStore {
             config.Confirmations ??= new ConfirmationSettings();
             return config;
         } catch (JsonException) {
-            // Corrupt: keep a copy for inspection, then continue with defaults.
-            File.Copy(FilePath, FilePath + ".bak", overwrite: true);
+            // Corrupt: keep a copy for inspection (best effort), then continue with defaults.
+            try {
+                File.Copy(FilePath, FilePath + ".bak", overwrite: true);
+            } catch (Exception exception) when (exception is IOException or UnauthorizedAccessException) {
+                // The backup is only a diagnostic aid; starting with defaults matters more.
+            }
             return new AppConfig();
         }
     }