Fix app hang during date formatting by pre-warming locale cache

Sources/RepoPrompt/App/RepoPromptApp.swift

@@ -69,6 +69,12 @@ struct RepoPromptApp: App {
         // Avoid process-killing SIGPIPE when the child closes stdin while we're still writing.
         signal(SIGPIPE, SIG_IGN)
 
+        // Pre-warm date formatter cache on a background thread to avoid blocking the main
+        // thread with ICU locale processing (ulocimp_addLikelySubtags) on first render.
+        Task.detached(priority: .utility) {
+            MessageTimestampFormatter.warmUp()
+        }
+
         SentryTelemetryBootstrap.start()
 
         ProcessDebugLogging.log(

Sources/RepoPrompt/Infrastructure/UI/Components/MessageTimestampFormatter.swift

@@ -6,6 +6,21 @@ import Foundation
 enum MessageTimestampFormatter {
     private static let cache = MessageTimestampFormatterCache()
 
+    /// Pre-warms the formatter cache on a background thread so that the first call from
+    /// the main thread (during SwiftUI rendering) does not block on expensive ICU locale
+    /// processing (ulocimp_addLikelySubtags / setLocalizedDateFormatFromTemplate).
+    static func warmUp() {
+        let calendar = Calendar.current
+        let locale = Locale.current
+        let now = Date()
+        // Touch every format / template variant the formatter can produce.
+        _ = cache.string(from: now, format: "HH:mm:ss", calendar: calendar, locale: locale)
+        for template in ["EEE", "MMM d", "MMM d y"] {
+            _ = cache.string(from: now, template: template, calendar: calendar, locale: locale)
+        }
+        _ = cache.relativeDayLabel(-1, calendar: calendar, locale: locale)
+    }
+
     static func string(
         from date: Date,
         includeDateContext: Bool,