Release v0.2.6 default-handler migration

[terrorproforma]

What changed

• auto-repair Explore Better-owned legacy PowerShell folder/drive handlers on packaged startup
• preserve an existing Explore Better default without prompting again
• repair context-menu-only installs without taking over a foreign default
• add live HKCU migration coverage and preserve the v0.2.5 website snapshot

Verification

• full acceptance: 131 pass, 0 fail
• current-user shell migration: 15 pass, 0 fail
• default-app UI: 15 pass, 0 fail
• release readiness: 31 pass, 0 fail
• speed health: 175 pass, 0 fail
• installed v0.2.6 locally; deliberately seeded the legacy command and confirmed first launch repaired all four handlers while Directory and Drive remained ExploreBetter