Implement minimal Pomodoro timer UI with countdown logic

[Copilot]

Implements a 25-minute Pomodoro timer with circular progress visualization and basic controls (start/stop/reset).

Implementation

• timer.html: SVG-based circular progress bar, time display (MM:SS), control buttons
• timer.css: Gradient UI with CSS variables for maintainability, state-based styling (working/stopped)
• timer.js: Class-based timer logic with 1-second countdown, progress calculation, Web Audio API notification

Technical Details

// Reuses AudioContext to avoid resource leaks
constructor() {
  this.audioContext = new (window.AudioContext || window.webkitAudioContext)();
  // ... setup
}

// Progress calculation drives SVG stroke-dashoffset
updateProgress() {
  const progress = this.timeRemaining / this.initialTime;
  const offset = this.circumference * (1 - progress);
  this.progressCircle.style.strokeDashoffset = offset;
}

CSS variables enable computed circumference:

:root {
  --progress-circumference: 817;
}
.progress-ring-circle {
  stroke-dasharray: var(--progress-circumference);
}

Screenshots

Initial state (stopped)

Running state with countdown

Original prompt

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This section details on the original issue you should resolve

<issue_title>ステップ1: 最小構成のタイマーUIとロジック</issue_title>
<issue_description>## 概要
最小構成のタイマーUIとロジックを実装する

タスク

• HTML/CSS/JSで静的なタイマー画面（円形プログレスバー、25:00表示、開始・リセットボタン）を作成
• タイマーのカウントダウンロジック（JavaScript）を実装
• ボタンでタイマーの開始・リセットができる
• 状態表示（作業中/停止中）

参照

plan.md のステップ1に基づく実装</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ステップ1: 最小構成のタイマーUIとロジック #57

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