Fix dark-mode clock icon and use macOS-style carets

[gusaiani]

Summary

• Fix clock icon in time/date inputs being invisible in dark mode by setting color-scheme: dark
• Replace all ▶/▼ unicode disclosure triangles with CSS-drawn chevron carets (border-based) that rotate smoothly on expand — matching macOS disclosure style
• Consolidate six duplicate chevron CSS blocks into one shared rule

Test plan

• Toggle dark mode and edit a session's start/end time — clock icon should be visible
• Expand/collapse task sessions, days, weeks, later section, and total row — carets should animate between right-pointing and down-pointing
• Verify play/promote buttons still show ▶ (not carets)