diff --git a/src/LageBuch.App/Services/SystemAlarmService.cs b/src/LageBuch.App/Services/SystemAlarmService.cs
index 4f08074..f316b09 100644
--- a/src/LageBuch.App/Services/SystemAlarmService.cs
+++ b/src/LageBuch.App/Services/SystemAlarmService.cs
@@ -120,6 +120,7 @@ private void PlayBlocking(AlarmSound sound, byte[] bytes)
         }
     }
 
+    [DefaultDllImportSearchPaths(DllImportSearchPath.System32)]
     [DllImport("winmm.dll", CharSet = CharSet.Auto)]
     private static extern bool PlaySound(byte[]? data, IntPtr hModule, uint flags);
 }
diff --git a/src/LageBuch.AppLogic/Services/IncidentStore.cs b/src/LageBuch.AppLogic/Services/IncidentStore.cs
index 270baf5..470acb8 100644
--- a/src/LageBuch.AppLogic/Services/IncidentStore.cs
+++ b/src/LageBuch.AppLogic/Services/IncidentStore.cs
@@ -5,7 +5,7 @@ namespace LageBuch.AppLogic.Services;
 
 public sealed class IncidentStore : IIncidentStore
 {
-    private readonly IIncidentFileStore _fileStore = new IncidentFileStore();
+private readonly IncidentFileStore _fileStore = new IncidentFileStore();
 
     public void Save(string path, Incident incident) => IncidentRepository.Save(path, incident);
 
diff --git a/src/LageBuch.AppLogic/ViewModels/FilesViewModel.cs b/src/LageBuch.AppLogic/ViewModels/FilesViewModel.cs
index 2590df1..fcc9249 100644
--- a/src/LageBuch.AppLogic/ViewModels/FilesViewModel.cs
+++ b/src/LageBuch.AppLogic/ViewModels/FilesViewModel.cs
@@ -154,13 +154,13 @@ private async Task OpenFileAsync(IncidentFileRow row)
 
     // Mirrors IncidentFile.AllowedContentTypes' extensions — the picker already restricts choice to
     // these, this just maps the chosen local path back to the MIME type the domain expects.
-    private static string ContentTypeFor(string path) => Path.GetExtension(path).ToLowerInvariant() switch
+    private static string ContentTypeFor(string path) => Path.GetExtension(path).ToUpperInvariant() switch
     {
-        ".jpg" or ".jpeg" => "image/jpeg",
-        ".png" => "image/png",
-        ".gif" => "image/gif",
-        ".webp" => "image/webp",
-        ".pdf" => "application/pdf",
+        ".JPG" or ".JPEG" => "image/jpeg",
+        ".PNG" => "image/png",
+        ".GIF" => "image/gif",
+        ".WEBP" => "image/webp",
+        ".PDF" => "application/pdf",
         _ => "application/octet-stream"
     };
 }
diff --git a/src/LageBuch.AppLogic/ViewModels/MainWindowViewModel.cs b/src/LageBuch.AppLogic/ViewModels/MainWindowViewModel.cs
index 7759419..2e95471 100644
--- a/src/LageBuch.AppLogic/ViewModels/MainWindowViewModel.cs
+++ b/src/LageBuch.AppLogic/ViewModels/MainWindowViewModel.cs
@@ -29,9 +29,9 @@ public MainWindowViewModel(HomeViewModel home, MasterDataEditorViewModel editor,
     // away (IncidentWorkspaceViewModel.GoHomeRequested). LeaveAsync is a no-op for a local session.
     private void ShowWorkspace(IncidentWorkspaceViewModel ws)
     {
-        ws.GoHomeRequested = () =>
+        ws.GoHomeRequested = async () =>
         {
-            _ = ws.LeaveAsync();
+            await ws.LeaveAsync();
             CurrentView = _home;
         };
         CurrentView = ws;
diff --git a/src/LageBuch.Domain/Atemschutz/AtemschutzTrupp.cs b/src/LageBuch.Domain/Atemschutz/AtemschutzTrupp.cs
index a8f68a4..509d13f 100644
--- a/src/LageBuch.Domain/Atemschutz/AtemschutzTrupp.cs
+++ b/src/LageBuch.Domain/Atemschutz/AtemschutzTrupp.cs
@@ -160,7 +160,7 @@ public static bool IsLpaTrupp(string designation) =>
     public static int RequiredMemberCount(string designation) =>
         IsChemicalTrupp(designation) ? ChemicalMemberCount : StandardMemberCount;
 
-    private static void ValidateCrew(string designation, IReadOnlyList<TruppMember> crew)
+    private static void ValidateCrew(string designation, List<TruppMember> crew)
     {
         // Atemschutz is never a solo activity -- a Trupp is the unit that goes under air together,
         // and the monitoring sheet has no concept of a single wearer. Enforcing the count here
diff --git a/src/LageBuch.Persistence/MasterData/MasterDataSet.cs b/src/LageBuch.Persistence/MasterData/MasterDataSet.cs
index 46ecbb5..124ba2b 100644
--- a/src/LageBuch.Persistence/MasterData/MasterDataSet.cs
+++ b/src/LageBuch.Persistence/MasterData/MasterDataSet.cs
@@ -231,6 +231,12 @@ public sealed record MasterDataSet(
 /// </summary>
 public static class MasterDataJson
 {
+    private static readonly JsonSerializerOptions ExportOptions = new()
+    {
+        WriteIndented = true,
+        Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
+    };
+
     public static MasterDataSet Parse(Stream json)
     {
         using var doc = JsonDocument.Parse(json);
@@ -400,10 +406,6 @@ public static string Serialize(MasterDataSet set)
             },
         };
 
-        return JsonSerializer.Serialize(model, new JsonSerializerOptions
-        {
-            WriteIndented = true,
-            Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
-        });
+        return JsonSerializer.Serialize(model, ExportOptions);
     }
 }
diff --git a/src/LageBuch.Persistence/MasterData/MasterDataStore.cs b/src/LageBuch.Persistence/MasterData/MasterDataStore.cs
index 2a598ea..285512e 100644
--- a/src/LageBuch.Persistence/MasterData/MasterDataStore.cs
+++ b/src/LageBuch.Persistence/MasterData/MasterDataStore.cs
@@ -93,7 +93,7 @@ private static void InsertChecklistTemplate(
         }
     }
 
-    private static IEnumerable<(string Key, int Value)> SettingsRows(IncidentSettings s) => new[]
+    private static (string Key, int Value)[] SettingsRows(IncidentSettings s) => new[]
     {
         ("ils_reminder_interval_minutes", s.IlsReminderIntervalMinutes),
         ("ils_reminder_follow_up_interval_minutes", s.IlsReminderFollowUpIntervalMinutes),
diff --git a/src/LageBuch.Sync/SyncCommand.cs b/src/LageBuch.Sync/SyncCommand.cs
index 29ba013..6774278 100644
--- a/src/LageBuch.Sync/SyncCommand.cs
+++ b/src/LageBuch.Sync/SyncCommand.cs
@@ -1,3 +1,4 @@
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json.Serialization;
 using LageBuch.Domain.CoMeasurement;
 using LageBuch.Domain.Etb;
@@ -113,6 +114,7 @@ public sealed record CloseIncidentCommand(OperatorDto Operator) : SyncCommand;
 
 // Bytes ride this one-shot upload command (System.Text.Json base64-encodes byte[] automatically),
 // but never the broadcast snapshot that follows every command — see IncidentSnapshot/IncidentFileDto.
+[SuppressMessage("Performance", "CA1819", Justification = "byte[] is the wire representation System.Text.Json base64-encodes; a read-only wrapper would need a custom converter.")]
 public sealed record AddFileCommand(OperatorDto Operator, string FileName, string ContentType, byte[] Bytes) : SyncCommand;
 
 // No operator on the wire: renaming is a silent label correction (no ETB entry), unlike every