Booking page: explain empty slot lists and minimum-notice gaps #20

From #14. Independent of the greyed-slots work in #19 and worth doing regardless.

Two places where the booking page withholds an explanation it already has.

1. The empty / sparse slot list

fillSlots renders No available times. when a day has none. It reads like a broken page rather than an answer, and gives no hint whether to try another day.

Something that names the date, and ideally the host, would land better. Key is no_available_times in internal/i18n/locales/*.json.

2. Minimum notice is invisible

If an event type sets min_notice_minutes, today's early slots silently vanish. A booker looking at today sees nothing until the evening with no explanation anywhere on the page. This is the single most likely cause of "why can't I see those times", and unlike the host's calendar it is policy, not private data, so it is safe to state plainly.

Show something like "Bookings require 4 hours' notice" when the notice rule is what removed the near-term slots.

Notes

• Three surfaces: book.html, manage.html, embed.js. Markup and JS are separate per surface; styling is shared in booking.css.
• New strings need adding to all eight locales in internal/i18n/locales/. The same-keys guard test will fail otherwise: go test ./internal/i18n/.
• Non-English locales are LLM drafts with no native review; that's the existing standard, no need to solve it here.