## Story 001: Focus session with visual countdown

**Persona:** Marcus, 28, software engineer, uses timers to protect deep work from Slack notifications. Knows Pomodoro exists but has never stuck with a timer app before.

**Situation:**

Marcus sits down to code. He knows he has 25 minutes before the next meeting. He opens the app. He wants to start focusing immediately without fiddling.

**Need:**

As Marcus, I want to start a focus session with one tap and see time counting down visually, so that I can trust the timer and stop checking my clock.

**Acceptance:**
- Tapping 'Start' begins a 25-minute session immediately (no configuration required)
- Time remaining is visible at all times in large, easy-to-read format
- A clear signal (sound, or visual, or both) marks the end of the session
- I can pause and resume within a session without losing my place

**Tier:** core

**Confusion-flags:**
- What happens if Marcus walks away mid-session? Do we pause automatically, or keep counting?
- Does the app need to work in the background, or is it foreground-only?
- Is there a visual distinction between 'paused' and 'running' that Marcus will instantly understand?
