Add MailRootView and related components for mail functionality

- Implement MailRootView with navigation and sidebar for mail management.
- Create MailSidebarView, ThreadListView, and ThreadDetailView for displaying mail content.
- Introduce ComposeView for composing new messages.
- Add MailTheme for consistent styling across mail components.
- Implement adaptive layouts for iOS and macOS.
- Create unit tests for AppNavigationCommand and AppViewModel to ensure correct functionality.

swift/Sources/App/AppControlService.swift

@@ -0,0 +1,66 @@
+import Foundation
+
+protocol AppControlServicing {
+    func commands() -> AsyncStream<AppNavigationCommand>
+}
+
+struct MockBackendControlService: AppControlServicing {
+    static let controlFileEnvironmentKey = "SOCIALIO_CONTROL_FILE"
+    static let pollingIntervalEnvironmentKey = "SOCIALIO_CONTROL_POLL_MS"
+
+    private let environment: [String: String]
+
+    init(environment: [String: String] = ProcessInfo.processInfo.environment) {
+        self.environment = environment
+    }
+
+    func commands() -> AsyncStream<AppNavigationCommand> {
+        guard let controlFilePath = environment[Self.controlFileEnvironmentKey]?
+            .trimmingCharacters(in: .whitespacesAndNewlines),
+            !controlFilePath.isEmpty else {
+            return AsyncStream { continuation in
+                continuation.finish()
+            }
+        }
+
+        let controlFileURL = URL(fileURLWithPath: controlFilePath)
+        let pollingInterval = pollingIntervalDuration
+
+        return AsyncStream { continuation in
+            let task = Task.detached(priority: .background) {
+                var lastAppliedPayload: String?
+
+                while !Task.isCancelled {
+                    if let payload = try? String(contentsOf: controlFileURL, encoding: .utf8) {
+                        let trimmedPayload = payload.trimmingCharacters(in: .whitespacesAndNewlines)
+
+                        if !trimmedPayload.isEmpty,
+                           trimmedPayload != lastAppliedPayload,
+                           let command = AppNavigationCommand.parse(trimmedPayload) {
+                            lastAppliedPayload = trimmedPayload
+                            continuation.yield(command)
+                        }
+                    }
+
+                    try? await Task.sleep(for: pollingInterval)
+                }
+
+                continuation.finish()
+            }
+
+            continuation.onTermination = { _ in
+                task.cancel()
+            }
+        }
+    }
+
+    private var pollingIntervalDuration: Duration {
+        guard let rawValue = environment[Self.pollingIntervalEnvironmentKey],
+              let milliseconds = Int(rawValue),
+              milliseconds > 0 else {
+            return .milliseconds(600)
+        }
+
+        return .milliseconds(milliseconds)
+    }
+}