fix: calculate track day from conference start date

[Link1515]

closes #315

Summary

修正僅在活動第二天有議程的 track，會被錯誤標示為「Day 1」的問題。Day 編號現在依 COSCUP 2026 的會期首日計算，而非依該 track 自身的日期列表排序。

Changes

• 在 track 頁面定義會期首日 2026-08-08。
• 以選取日期與會期首日的日曆差計算 Day 編號。
• 保留未選取日期時顯示 Day 1 的既有預設行為。

Summary by CodeRabbit

• Bug Fixes
  • Corrected conference day selection to align with the calendar date.
  • Defaults to the first conference day when no day is selected.

[coderabbitai]

📝 Walkthrough

Walkthrough

The track page now uses a fixed conference start date to calculate the selected calendar day. It returns day 1 when no day is selected.

Changes

Conference day index

Layer / File(s)	Summary
Date-based day calculation app/pages/track/[id].vue	The page defines CONFERENCE_START_DAY and calculates dayIndex with timezone-aware date arithmetic. Empty selections still return day 1.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related issues

• UX enhancement: [議程表] 只有在第二天出現的議程軌，於單日檢視的情況下 #268 — The updated dayIndex calculation changes the day label shown on single-day track views.
• UX enhancement: [議程表] 跨日議程軌，從第二日點擊進入後應該直接顯示第二日的議程 #267 — The track page now derives the displayed day from the fixed conference date.
• 只有 Day 2 議程軌頁面會顯示 Day 1 #315 — The change corrects day labels that can show Day 1 for Day 2 tracks.

Possibly related PRs

• COSCUP/2026#215 — Both PRs modify the day index calculation in app/pages/track/[id].vue.

Suggested reviewers: mirumodapon, pan93412

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: calculating the track day from the conference start date.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch track-day

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[coderabbitai]

🧹 Nitpick comments (1)

app/pages/track/[id].vue (1)

113-119: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Add regression tests for the day-index contract.

Cover 2026-08-08 → 1, 2026-08-09 → 2, and an empty selection → 1. These cases protect the fixed-date calculation and fallback behavior.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@app/pages/track/`[id].vue around lines 113 - 119, Add regression tests for
the day-index calculation used by the selectedDay logic, covering 2026-08-08
returning 1, 2026-08-09 returning 2, and an empty selection returning 1. Reuse
the existing test setup and target the component’s day-index behavior without
changing the implementation.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@app/pages/track/`[id].vue:
- Around line 113-119: Add regression tests for the day-index calculation used
by the selectedDay logic, covering 2026-08-08 returning 1, 2026-08-09 returning
2, and an empty selection returning 1. Reuse the existing test setup and target
the component’s day-index behavior without changing the implementation.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: bf242105-cca4-400b-8ed6-6c8421499fa6

📥 Commits

Reviewing files that changed from the base of the PR and between 0d49932 and 8b837aa.

📒 Files selected for processing (1)

• app/pages/track/[id].vue