Improve moon-phase rendering: smooth offsets, finer Chinese labels, and live updates

[Heersin]

Motivation

• Make moon visualization reflect the lunar cycle more smoothly and provide more granular Chinese phase labels for better UX.
• Ensure displayed Gregorian and Chinese lunar datetimes (including traditional earthly-branch hours) stay up to date.
• Replace one-off computation with a periodic renderer so the UI updates continuously without reloads.

Description

• Reworked assets/js/moon-phase.js to move DOM lookups, introduce a render function, and drive updates via setInterval calling render(Date.now()) every second.
• Replaced the coarse four-state phase logic with a finer-grained getMoonPhaseLabel mapping and a smooth --phase-offset calculation that interpolates across the synodic month for gradual light/dark transitions.
• Restored and localized date/Chinese lunar formatting using Intl.DateTimeFormat, and recompute the traditional earthlyBranches hour label each render.
• Preserved guards for absent DOM elements and update text content, aria-label, and CSS variable from the render loop.

Testing

• No automated tests were run for this change.

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Codex Task