fix: improve keystroke performance with debounced saves

[farseenmanekhan1232]

Problem

Users were experiencing frame drops and delays on every keystroke.

Root Cause

The app was performing synchronous disk writes on every single character typed. Each keystroke triggered:

1. File write to disk (blocking main thread)
2. File read to update preview (more blocking I/O)
3. State mutation causing view re-render

Changes

• Debounce text saves (500ms delay) instead of saving on every keystroke
• Move file I/O to background queue to avoid blocking main thread
• Add updatePreviewTextFromContent to avoid re-reading saved file
• Add willResignActiveNotification handler to save when app loses focus
• Remove unused saveTimer that was creating unnecessary overhead

Testing

Tested locally - no more frame drops during rapid typing. Text is still reliably saved after 500ms of inactivity or when switching apps.