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/plan.md

@@ -0,0 +1,74 @@
+# Implementation Plan: Copy URL & Toast Notifications
+
+**Branch**: `016-copy-url-toast` | **Date**: 2026-05-09 | **Spec**: [spec.md](spec.md)
+**Input**: Feature specification from `specs/016-copy-url-toast/spec.md`
+
+## Summary
+
+Add a "Copy URL" button to the image detail page that copies the image's direct file URL to the clipboard, with a reusable toast notification service wired to confirm success or failure. All changes are UI-only; no API changes are required.
+
+## Technical Context
+
+**Language/Version**: TypeScript (strict mode), Angular latest stable
+**Primary Dependencies**: Angular (`@angular/core`, `@angular/common`), RxJS (`BehaviorSubject`), browser Clipboard API (`navigator.clipboard.writeText`)
+**Storage**: N/A
+**Testing**: Karma/Jasmine (`ng test`)
+**Target Platform**: Browser (modern; Clipboard API requires HTTPS — already in place)
+**Project Type**: Angular standalone SPA
+**Performance Goals**: Copy action completes in < 100ms perceived latency; toast appears within 300ms of action
+**Constraints**: TypeScript strict mode, `ChangeDetectionStrategy.OnPush` on all components, no new npm dependencies
+**Scale/Scope**: Two new files (service + component), two modified files (detail + app component)
+
+## Constitution Check
+
+*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*
+
+| Principle | Status | Notes |
+|-----------|--------|-------|
+| §2.1 Strict separation of concerns | ✓ PASS | Pure UI change; no API knowledge in UI beyond what's already in `ImageRecord.file_url` |
+| §2.6 No speculative abstraction | ✓ PASS | Toast service is justified: used immediately by this feature and explicitly planned for reuse (upload confirmation, delete confirmation, filter feedback). Two concrete use cases exist. |
+| §5.1 Tests alongside implementation | ✓ PASS | Tests required for `ToastService` and the copy button on `DetailComponent` |
+| §5.2 Test pyramid | ✓ PASS | Unit tests only (no API/DB involved); Karma/Jasmine |
+| §6 Tech stack | ✓ PASS | Angular, TypeScript strict — no new dependencies |
+| §7.3 Linting | ✓ PASS | `ng lint` must pass before task is done |
+| §8 Scope boundaries | ✓ PASS | No multi-user, no embeds, no public sharing infrastructure — just a clipboard copy |
+
+**Post-design re-check**: No violations. Feature is entirely additive.
+
+## Project Structure
+
+### Documentation (this feature)
+
+```text
+specs/016-copy-url-toast/
+├── plan.md              ← this file
+├── research.md          ← Phase 0 output
+├── quickstart.md        ← Phase 1 output
+├── contracts/
+│   └── toast-service.md ← Phase 1 output
+└── tasks.md             ← /speckit-tasks output
+```
+
+### Source Code
+
+```text
+ui/src/app/
+├── app.component.ts              ← modified: add <app-toast> to template
+├── services/
+│   └── toast.service.ts          ← new: singleton toast service
+├── toast/
+│   └── toast.component.ts        ← new: toast display component
+└── detail/
+    └── detail.component.ts       ← modified: add Copy URL button + inject ToastService
+
+ui/src/app/services/
+    toast.service.spec.ts         ← new: unit tests for ToastService
+
+ui/src/app/toast/
+    toast.component.spec.ts       ← new: unit tests for ToastComponent
+
+ui/src/app/detail/
+    detail.component.spec.ts      ← modified: tests for copy button behaviour
+```
+
+**Structure Decision**: Single-project Angular SPA. Toast service lives in `services/` alongside `ImageService` and `TagService`. Toast component gets its own `toast/` directory following the existing component-per-directory convention.