feat: add track (議程軌) row pinning

[rileychh]

依 Figma 新設計為桌面版「議程軌（社群）模式」議程表加入議程軌釘選（pin）功能，並預設釘選主議程。此為 #238 的後續，基底為 session-track-mode 分支。

變更內容

• 議程軌欄的釘選圖示改為可互動的切換按鈕（取代原本的靜態圖示），點擊即釘選／取消釘選該議程軌。
• 預設釘選「主議程」（Main Session Track）；以議程軌名稱比對（同時支援中英文名稱），不綁定會變動的 Pretalx track id。
• 釘選的議程軌移至列表頂端；釘選群組依「釘選順序」排列，後釘選者排在最後。
• 每個議程軌的顏色依 track id 順序固定，不受釘選重新排序影響。
• 釘選狀態以 localStorage 保存並跨重新載入；區分「從未造訪」（預設釘選主議程）與「使用者取消全部釘選」（保留空清單）。
• 釘選列樣式依 Figma：藍色左側強調條（4px #3b82f6）、blue-50 底色、藍色軌名／教室代碼、填色釘選按鈕；未釘選列維持預設中性樣式。
• 釘選按鈕 hover 顯示 pointer 游標。

Summary by CodeRabbit

• New Features

  • Added per-track pinning in the session track table, letting users keep important tracks at the top.
  • Track pin state is now saved between visits, so pinned tracks persist across sessions.
  • The main session track is automatically pinned on first visit.

• UI Improvements

  • Pinned tracks now have clearer visual emphasis.
  • The pin control is now interactive, with updated labels for better accessibility.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch session-track-pinning

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[coderabbitai]

Actionable comments posted: 1

🤖 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.

Inline comments:
In `@app/components/feature/CpSessionTrackTable.vue`:
- Around line 134-140: The first-visit default pin in CpSessionTrackTable is
being derived from the current day-filtered tracks list, so it can save an empty
pinned state before the main track appears. Update the initialization logic in
the watch(tracks, ...) block to use an event-wide track source instead of
tracks.value when choosing the default main track, so the initial pinnedKeys is
set correctly regardless of selectedDay changes. Keep the persisted
coscup-pinned-tracks behavior, but make the default selection independent of the
first loaded day.

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Fix all unresolved CodeRabbit comments on this PR:

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• app/components/feature/CpSessionTrackTable.vue