Remember last save directory across file operations

[sensei-hacker]

Summary

Implement persistent "last save directory" feature that remembers the directory used in file save dialogs and automatically defaults to it in future save operations.

Problem

Currently, save dialogs always default to some system directory (Documents, Downloads, etc.). Users must navigate to their preferred location every single time they save a file - whether it's blackbox logs, diffs, configurations, or other exports. This is particularly frustrating when saving multiple files in a session.

Solution

Modified showSaveDialog() wrapper to:

1. Retrieve the last saved directory from electron-store
2. Set it as defaultPath for the save dialog (if no path was explicitly provided)
3. After successful save, extract the directory from the returned filepath
4. Store it for next time

Changes

File Modified: js/dialog.js

• Added LAST_SAVE_DIRECTORY_KEY constant for storage key
• Enhanced showSaveDialog() to get/set last directory
• Cross-platform path handling (supports both / and \ separators)
• Uses existing electronAPI.storeGet/storeSet infrastructure

Benefits

• Reduces friction: Users don't have to navigate to the same directory repeatedly
• Persists across sessions: Works even after app restart
• Universal: Applies to ALL save operations automatically (blackbox, diffs, configs, etc.)
• Transparent: No UI changes - just works
• Backwards compatible: Gracefully handles first use (no saved directory yet)
• Non-intrusive: Doesn't override explicitly provided defaultPath

Testing

• Tested save dialog flow
• Verified directory extraction logic
• Checked storage persistence
• Windows path handling should be tested

Use Cases

This improves UX for common workflows:

• Saving multiple blackbox logs to analysis directory
• Exporting configuration diffs to backup location
• Saving CLI dumps to troubleshooting folder