Add project guidelines and README for app overview and usage instructions

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`.