Cleanup: Remove unused InstallerView.swift

[malpern]

Problem

InstallerView.swift is legacy dead code showing "not yet implemented" message. The actual Installation Wizard is fully implemented in InstallationWizard/ directory with 30+ files.

Location

Sources/KeyPathAppKit/UI/InstallerView.swift

Impact

Confusing for code reviewers, adds maintenance burden.

Fix

1. Remove InstallerView.swift
2. Verify no references to it
3. Document that InstallationWizard/ is the real implementation

References

• Comprehensive review: docs/code-review/comprehensive-feature-review.md section 10