Consolidate notification fixes and performance optimizations

[google-labs-jules]

Consolidated notification-related fixes and performance optimizations into a single branch.

Key Changes:

• Menu: Enabled "Show Notifications", "Show Message Preview", and "Show Unread Badge" for all users (removed is.development checks). Removed duplicate menu items.
• Logic: Updated source/browser/conversation-list.ts to respect notificationsMuted config by suppressing the notification IPC call, while continuing to track unread messages to prevent state desynchronization.
• Performance: Refactored isUnread to use TreeWalker instead of querySelectorAll for more efficient DOM traversal.
• Reactivity: Added listeners in source/index.ts to update the unread badge immediately upon config change.
• Flash Window: Added window flashing logic for non-macOS systems when a message arrives.

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PR created automatically by Jules for task 7246769853686056226 started by @raztronaut

[google-labs-jules]

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