fix: gallery import launches ML Kit scanner for editing

User wants the ML Kit edit screen (crop/perspective/filter) for
gallery photos. ML Kit only edits gallery images via its in-scanner
gallery icon, so the 갤러리 불러오기 button now opens the same ML Kit
scanner as camera. Removes the photo-picker→result path and the now-
unused GalleryImporter + its test + import_failed string.

Constraint: ML Kit has no API to feed a gallery bitmap directly to its editor.
Confidence: high
Scope-risk: local
Reversibility: trivial
Directive: user verifies gallery icon→edit flow on device.
Tested: clean build + 6 unit tests pass.
Not-tested: on-device scanner gallery flow.
Related: 2026-05-30-clearcam-home-camgallery-design.md

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