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

[sentry]

This PR addresses an app hanging issue (REPOPROMPT-8) where the application would freeze for 2000ms+ on macOS 26.x.

Root Cause:
The hang was traced to synchronous, expensive ICU (International Components for Unicode) locale processing occurring on the main thread. Specifically, the MessageTimestampFormatter's setLocalizedDateFormatFromTemplate(_:) call, triggered during SwiftUI rendering, would block the main thread while inferring locale subtags (ulocimp_addLikelySubtags -> RegionValidateMap::equals -> res_getTableItemByKey). This was particularly noticeable on macOS 26.x, suggesting changes in ICU data loading behavior.

Solution:
To prevent this main thread blockage, the MessageTimestampFormatter's cache is now pre-warmed on a background thread during application startup. This ensures that when the main thread first requests a formatted timestamp, the expensive DateFormatter and RelativeDateTimeFormatter instances, along with their underlying ICU data, have already been initialized and cached.

Changes Made:

• Added a warmUp() static method to MessageTimestampFormatter to explicitly pre-populate its internal DateFormatter and RelativeDateTimeFormatter caches for all commonly used formats and templates.
• Integrated a call to MessageTimestampFormatter.warmUp() within a Task.detached(priority: .utility) in RepoPromptApp.init(). This executes the pre-warming on a background thread, preventing UI freezes.

The existing NSLock in MessageTimestampFormatterCache ensures thread-safe access during concurrent initialization and subsequent main-thread usage.

Fixes REPOPROMPT-8