Implement watch-event feature (017) with bookmark in RsvpBar

Add client-side watch/bookmark functionality: users can save events to
localStorage without RSVPing via a bookmark button next to the "I'm attending"
CTA. Watched events appear in the event list with a "Watching" label.
Bookmark is only visible for visitors (not attendees or organizers).

Includes spec, plan, research, tasks, unit tests, and E2E tests.

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

specs/017-watch-event/plan.md

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+# Implementation Plan: Watch Event
+
+**Branch**: `017-watch-event` | **Date**: 2026-03-12 | **Spec**: [spec.md](spec.md)
+**Input**: Feature specification from `/specs/017-watch-event/spec.md`
+
+## Summary
+
+Add a bookmark icon to the event detail page (left of event title) that lets users save events to localStorage without RSVPing. Watched events appear in the event list with a "Watching" label. The feature is entirely client-side — no backend changes required.
+
+## Technical Context
+
+**Language/Version**: TypeScript 5.9 (frontend only)
+**Primary Dependencies**: Vue 3, Vue Router 5
+**Storage**: localStorage via `useEventStorage.ts` composable (existing)
+**Testing**: Vitest for unit tests, Playwright + MSW for E2E
+**Target Platform**: Mobile-first PWA (browser)
+**Project Type**: Web application (frontend-only change)
+**Performance Goals**: Instant toggle (no network requests)
+**Constraints**: No backend involvement, no new dependencies
+**Scale/Scope**: 4 modified files, 0 new files
+
+## Constitution Check
+
+*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*
+
+| Principle | Status | Notes |
+|-----------|--------|-------|
+| I. Privacy by Design | PASS | No data sent to server. Watch is purely localStorage. No tracking. |
+| II. Test-Driven Methodology | PASS | Unit tests for composable changes, E2E tests for user stories. |
+| III. API-First Development | PASS | No API changes needed. Feature is client-side only. |
+| IV. Simplicity & Quality | PASS | Minimal changes to existing code. No new abstractions. |
+| V. Dependency Discipline | PASS | No new dependencies introduced. |
+| VI. Accessibility | PASS | Bookmark icon will use semantic button/ARIA, keyboard-operable. |
+
+No violations. Gate passes.
+
+## Project Structure
+
+### Documentation (this feature)
+
+```text
+specs/017-watch-event/
+├── plan.md              # This file
+├── spec.md              # Feature specification
+├── research.md          # Phase 0 output
+├── data-model.md        # Phase 1 output
+├── quickstart.md        # Phase 1 output
+└── tasks.md             # Phase 2 output (/speckit.tasks)
+```
+
+### Source Code (files to modify)
+
+```text
+frontend/src/
+├── composables/
+│   ├── useEventStorage.ts        # Add saveWatch(), isStored() methods
+│   └── __tests__/
+│       └── useEventStorage.spec.ts  # Tests for new methods
+├── components/
+│   ├── EventCard.vue              # Add 'watcher' role + badge styling
+│   ├── EventList.vue              # Update getRole() to return 'watcher', adjust delete flow
+│   └── __tests__/
+│       ├── EventCard.spec.ts      # Tests for watcher badge
+│       └── EventList.spec.ts      # Tests for watcher delete behavior
+├── views/
+│   ├── EventDetailView.vue        # Add bookmark icon next to title
+│   └── __tests__/
+│       └── EventDetailView.spec.ts  # Tests for bookmark behavior
+└── e2e/
+    └── watch-event.spec.ts        # E2E tests for all user stories
+```
+
+**Structure Decision**: Frontend-only changes. No new files needed — all modifications go into existing components and composables. One new E2E test file.
+
+## Complexity Tracking
+
+No constitution violations. No complexity justifications needed.