FEATURE: Add auto-reload feature for unmodified files when they change on disk

[mikejoseph23]

Implements automatic reloading of unmodified markdown files when they change on disk, with a green success notification that auto-dismisses after 5 seconds with a countdown timer.

Changes:

• Added 'autoReloadUnmodifiedFiles' preference (default: true) in new Experimental section
• Created Experimental preferences UI component with beaker icon
• Modified file change detection to auto-reload unmodified files
• Enhanced notification system to support auto-hide with countdown timer
• Green success notification shows "Document automatically reloaded from disk (5s)"
• Blue confirmation prompt for modified files remains unchanged (no auto-dismiss)
• Fixed pushTabNotification to properly pass autoHide properties

[mikejoseph23]

Testing Instructions

Setup

1. Build and install the application from this branch
2. Open a markdown file in MarkText

Test Case 1: Auto-reload unmodified files (green notification)

1. Open a markdown file without making any changes
2. Using an external editor (e.g., VS Code, vim), modify the file and save it
3. Expected: Green notification appears saying "Document automatically reloaded from disk (5s)"
4. Expected: The notification should auto-dismiss after 5 seconds with a countdown timer
5. Expected: The file content should be automatically reloaded

Test Case 2: Modified files prompt (blue notification)

1. Open a markdown file and make some changes (do NOT save)
2. Using an external editor, modify the same file and save it
3. Expected: Blue notification appears asking "Do you want to reload it?" with an OK button
4. Expected: This notification should NOT auto-dismiss (stays until you click OK or X)
5. Click OK to reload or X to dismiss

Test Case 3: Experimental preferences UI

1. Open Preferences (MarkText > Preferences on Mac)
2. Navigate to the new "Experimental" section (should have a beaker/flask icon)
3. Expected: See the "Automatically reload unmodified documents when changed on disk" toggle
4. Expected: The toggle should be ON by default
5. Toggle it OFF and repeat Test Case 1
6. Expected: Blue confirmation prompt should appear instead of auto-reload

Test Case 4: Countdown timer

1. With the feature enabled, trigger an auto-reload (Test Case 1)
2. Expected: The countdown should display "(5s)" and count down: "(4s)" → "(3s)" → "(2s)" → "(1s)" → dismiss
3. Click the X button before the countdown expires
4. Expected: Notification should dismiss immediately and countdown should stop