Add project guidelines and README for app overview and usage instructions

.github/copilot-instructions.md

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+# Project Guidelines
+
+## Code Style
+- Keep components as functional React components and use hooks-based state.
+- Follow existing naming and structure in `src/screens` and `src/components`:
+  - Screen files: `*Screen.js`
+  - Shared UI pieces: `src/components/*.js`
+- Keep styling centralized in `src/styles.js` via `createStyles()` and use theme-driven inline color overrides in screens/components.
+- Reuse `getTheme(darkMode, pinkMode)` from `src/theme.js` for color values; do not hardcode alternate palettes in individual screens.
+
+## Architecture
+- Root orchestration lives in `App.js`:
+  - App-level state includes current screen, focus mode, theme toggles, and timer/countdown state.
+  - Screen switching is controlled by the `screen` state (`home`, `timeuntil`, `timer`).
+- Screen responsibilities:
+  - `HomeScreen`: mode selection and top controls.
+  - `TimeUntilScreen`: target clock-time countdown flow.
+  - `TimerScreen`: duration countdown flow.
+  - `FocusScreen`: minimal fullscreen countdown UI.
+- Shared presentational components:
+  - `TopControls`: dark/pink/fullscreen/focus controls.
+  - `CountdownRow`: reusable HH:MM:SS display.
+
+## Build And Run
+- Install dependencies: `npm install`
+- Start dev server: `npm start`
+- Run on Android: `npm run android`
+- Run on iOS: `npm run ios`
+- Run on web: `npm run web`
+- EAS builds use `eas.json` profiles (`development`, `preview`, `production`).
+
+## Conventions
+- Keep timer/countdown behavior in `App.js` unless intentionally refactoring architecture.
+- Preserve current time behavior:
+  - `now` updates every second with `setInterval`.
+  - Time-until target is based on local device time and rolls to next day when target time has passed.
+- Maintain platform guards:
+  - Web-only fullscreen uses `document.fullscreenElement` APIs.
+  - Android hardware back behavior is handled in `App.js` and should keep focus/screen fallback behavior.
+- For countdown display, continue using `CountdownRow` and 2-digit padded units for consistency.
+
+## Pitfalls
+- This project has no test scripts configured; do not claim tests were run unless you add and run them.
+- Be careful with web-only globals (`document`) and keep `Platform.OS` guards.
+- Avoid introducing timezone assumptions without explicit product requirements; current logic is local-time based.

README.md

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+# Time Until
+
+A small Expo React Native app with two countdown modes:
+
+- **Time Until**: count down to a target clock time (local device time)
+- **Timer**: count down from a duration you enter (hours/minutes/seconds)
+
+It also includes a minimal focus view, dark mode, and a pink accent theme.
+
+## Download
+
+You can download ready-to-use apk builds from the Releases page:
+
+https://gitea.reversed.dev/space/time-until/releases
+
+## How It Works
+
+- Main app state and countdown logic live in `App.js`.
+- Screen components are in `src/screens`.
+- Shared UI components are in `src/components`.
+- Theme + styling are managed via `src/theme.js` and `src/styles.js`.
+
+### Time Until Mode
+
+- Enter hour/minute in 24-hour format.
+- If the selected time has already passed today, it rolls to the next day.
+
+### Timer Mode
+
+- Enter a duration in hours/minutes/seconds.
+- Supports start, pause, resume, and reset.
+
+## Notes
+
+- Time calculations are based on local device time.
+- Web fullscreen uses browser fullscreen APIs.
+- No automated test scripts are currently configured in `package.json`.