Window Management: Add Accessibility permission pre-check

[malpern]

Problem

Window management won't work without Accessibility permission, but there's NO PRE-CHECK. Users get vague "Unable to move window" error.

Impact

• Users enable Window Snapping
• Try to use it
• Get generic error message
• No guidance about missing permission

Fix Needed

1. Check Accessibility permission at WindowManager init
2. Show permission dialog BEFORE user tries to use feature
3. Update WindowSnappingView to show permission status indicator
4. Provide clear error messages when permission missing

Location

• ActionDispatcher.handleWindow() line 503
• WindowManager initialization

References

• Comprehensive review: docs/code-review/comprehensive-feature-review.md
• docs/features/window-management.md