Feat: Add Copy URL button and reusable toast notification system

Detail page now has a "Copy URL" button that copies the image's direct
file URL to the clipboard. A toast service (BehaviorSubject-backed,
auto-dismissing after 3s) confirms success or failure. ToastComponent
is registered at the app root and available to all future features.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

specs/016-copy-url-toast/checklists/requirements.md

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+# Specification Quality Checklist: Copy URL & Toast Notifications
+
+**Purpose**: Validate specification completeness and quality before proceeding to planning
+**Created**: 2026-05-09
+**Feature**: [spec.md](../spec.md)
+
+## Content Quality
+
+- [X] No implementation details (languages, frameworks, APIs)
+- [X] Focused on user value and business needs
+- [X] Written for non-technical stakeholders
+- [X] All mandatory sections completed
+
+## Requirement Completeness
+
+- [X] No [NEEDS CLARIFICATION] markers remain
+- [X] Requirements are testable and unambiguous
+- [X] Success criteria are measurable
+- [X] Success criteria are technology-agnostic (no implementation details)
+- [X] All acceptance scenarios are defined
+- [X] Edge cases are identified
+- [X] Scope is clearly bounded
+- [X] Dependencies and assumptions identified
+
+## Feature Readiness
+
+- [X] All functional requirements have clear acceptance criteria
+- [X] User scenarios cover primary flows
+- [X] Feature meets measurable outcomes defined in Success Criteria
+- [X] No implementation details leak into specification
+
+## Notes
+
+- All items pass. Ready to proceed to `/speckit-plan`.