Fix 'General' settings not available after upgrade (blocks startup)

[chelslava]

Summary

• Fixes critical bug where RIDE fails to start after upgrade when 'General' settings section is missing
• Adds settings migration from version 8 to 9 that ensures General section exists with all required keys
• Adds safe access to General settings in multiple files using .get() with fallback values

Problem

After upgrading RIDE, the application crashes on startup with:

KeyError: 'General'
OnInit returned false, exiting...

This happens when:

1. User's settings file is corrupted or incomplete
2. Settings migration from older versions doesn't create the General section
3. Direct dictionary access settings['General'] fails

Root Cause

Multiple files accessed settings['General'] directly without checking if the section exists:

• application.py:150 - self.settings['General'].get('font size', 12)
• mainframe.py:163 - application.settings['General']
• treeplugin.py:291 - settings.get_without_default('General')
• dialog.py:34 - _settings['General']

Solution

1. Added migration migrate_from_8_to_9() - Creates General section with default values if missing
2. Safe dictionary access - Changed all direct ['General'] access to .get('General', {}) with fallback values
3. Defensive coding - All General settings now have sensible defaults

Files Modified

• src/robotide/preferences/settings.py - Added migration method
• src/robotide/preferences/settings.cfg - Updated version to 9
• src/robotide/application/application.py - Safe access to font settings
• src/robotide/ui/mainframe.py - Safe access to general settings
• src/robotide/ui/treeplugin.py - Safe access to theme settings
• src/robotide/widgets/dialog.py - Safe access to dialog settings

Testing

• Tested by simulating missing General section in settings
• Application starts successfully with default values
• Existing settings are preserved and merged correctly

Closes #3000