feat: integrate settings and speed limiter into main window

- Refactor SettingsView to act as a flat detail pane instead of a nested sidebar or macOS native window
- Add custom horizontal tab bar for switching between setting categories
- Create SpeedLimiterView and add it as a primary tool in the sidebar
- Add state retention using AppStorage for settings tab selection and speed limits
- Fix EXC_BAD_ACCESS in DownloadTable by using sortableSize instead of debugDescription

Sources/Firelink/Aria2DownloadEngine.swift

@@ -301,7 +301,7 @@ final class Aria2DownloadEngine {
         var lines = [
             urls,
             "  dir=\(sanitizedOptionValue(item.destinationDirectory.path))",
-            "  out=\(sanitizedOptionValue(item.fileName))",
+            "  out=\(sanitizedOptionValue(item.fileName.replacingOccurrences(of: "/", with: "_").replacingOccurrences(of: "\\", with: "_")))",
             "  split=\(connections)",
             "  max-connection-per-server=\(connections)"
         ]