Prevent deletion of the personal project

[bborn]

Summary

• Added protection to prevent users from deleting the "personal" project
• The personal project is the default project automatically created during database initialization
• Attempting to delete it now shows a clear error message to the user

Changes

• Database layer: Modified DeleteProject() in internal/db/tasks.go to check if the project name is "personal" and return an error if deletion is attempted
• UI layer: Updated settings view in internal/ui/settings.go to properly handle and display deletion errors to users
• Tests: Added comprehensive test TestDeletePersonalProject() in internal/db/tasks_test.go to verify the protection works correctly

Test plan

• Run go test ./internal/db - all tests pass including new test
• Run go build ./... - project builds successfully
• Verify personal project cannot be deleted (returns error)
• Verify other projects can still be deleted normally
• Manual UI testing: Try to delete personal project in settings and verify error is displayed
• Manual UI testing: Verify deletion of other projects still works

Additional Notes

This fix ensures the personal project, which serves as the default workspace for tasks, cannot be accidentally deleted by users. The protection is implemented at the database layer for maximum safety.

Fixes #149

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