docs: add easter egg design spec

docs/superpowers/specs/2026-04-30-easter-egg-design.md

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+# Easter Egg — Fiat Code Reveal
+
+## Date
+2026-04-30
+
+## Status
+Approved
+
+---
+
+## Overview
+
+Add a hidden Easter egg to the TracPulse app that opens `https://fiatcode.dev` when triggered, preceded by a fun celebratory animation.
+
+---
+
+## Trigger
+
+- **Location:** `SettingsScreen`
+- **Gesture:** Triple-tap on the Settings screen `AppBar`
+- **Feedback:** None until triggered
+
+---
+
+## Animation Sequence
+
+1. A full-screen semi-transparent overlay appears over the Settings screen
+2. Animated "🎉 FIAT CODE! 🎉" text fades in with a scale-up effect
+3. Animation holds for ~1.5 seconds, then fades out
+4. The URL `https://fiatcode.dev` opens in the browser (via `url_launcher`)
+
+---
+
+## Technical Approach
+
+- **Overlay:** `Stack` widget layer above current screen content, managed via a stateful overlay widget
+- **Animation:** Flutter built-ins — `AnimatedOpacity` + `AnimatedScale` (no extra packages needed)
+- **URL Launch:** `url_launcher` package (check if already in `pubspec.yaml`; if not, add it)
+- **State:** Local state in `SettingsScreen` controls overlay visibility and animation
+- **One-shot:** Overlay auto-dismisses after animation completes; subsequent triple-taps re-trigger
+
+---
+
+## Components
+
+1. **`EasterEggOverlay` widget** — positioned widget with `AnimatedOpacity` + `AnimatedScale`, displays centered animated text, auto-dismisses via `Future.delayed`
+2. **Gesture detector** — `GestureDetector` with `onTap` counting triple-taps on the Settings `AppBar`
+3. **URL launcher call** — `await launchUrl(Uri.parse('https://fiatcode.dev'))` after animation completes
+
+---
+
+## Files Affected
+
+- `lib/settings_screen.dart` — add overlay, gesture detector, animation logic
+- `pubspec.yaml` — add `url_launcher` if not already present