feat: ship backup wizard and assistant-aware backup metadata for v1.2.0

test(backup-service): align essentials expectations with full workspace

refactor(updater): remove Sparkle updater service

fix(backup-service): treat workspaces as essentials in partial mode

refactor(settings): remove Sparkle update controls

feat(archives): add assistant and version filter cues

refactor(app): remove Sparkle updater injection

fix(paths): support agents.defaults workspace discovery

test(backup-service): add essentials backup path coverage

fix(restore): populate new backup request parameters

fix(import): preserve assistant product from archive metadata

feat(preview): display assistant product in archive summary

feat(backup-ui): add step-based backup wizard experience

feat(backup-service): support essentials backup and estimation

feat(backup-meta): add assistant product metadata field