UX: replace flat start menu with goal-based first-scan flow

[jonathanweinberg]

Summary

• replace the flat launcher menu with a six-goal, state-aware home screen
• simplify first scan to four decisions with safe defaults, Back/Cancel handling, and explicit saved-file replacement
• bind review/dashboard actions to the current validated export and restore saved workflow context on restart
• defer optional ownership work and offer an owner-mapping draft after successful validation
• update PS5.1 smoke coverage, regression tests, and operator documentation

Validation

• git diff --check
• changed-file PowerShell parser check plus forced module import
• pwsh -NoLogo -NoProfile -File tests/Invoke-ShareSurferTests.ps1 — 154/154 passed
• npm --prefix interface/standalone-dashboard run test — 57/57 passed
• npm --prefix interface/standalone-dashboard run build

Long logs were captured under /private/tmp/sharesurfer-validation-20260709-tui/.

Closes #429