Build the landing page with subscribe cards and next launch

[roma321m]

What

• npm run build renders src/page.html into site/index.html from src/calendars.ts (one card per calendar: Add to Google Calendar, Apple / Outlook via webcal://, copy ICS link) and writes site/next.json (next 5 launches).
• site/app.js: next launch panel with T-/T+ countdown in the visitor's time zone, moves to the next launch when one ends; copy button with clipboard and prompt fallback. Page works without JS apart from those two.
• SpaceX-inspired design (black, D-DIN, outline buttons), self-hosted D-DIN (OFL) and one NASA public-domain photo (CRS-20, credited, no endorsement implied). No third-party requests, no cookies; 272 KB total.

Why
Replaces the placeholder with the subscribe page for the 4 calendars.

Verification

• 71 tests green, typecheck clean; page renderer and nextLaunches assertions mutation-checked (one equivalent mutant: base64 vs base64url, identical for all 4 real ids).
• Local preview at 1600, 1280 and 375 px: no horizontal scroll, fonts load, all requests same-origin. Countdown verified live (Crew-13 "Just launched", T+) and by faking the clock past each launch (switches, then hides). Copy: success label and status, double-click restore, prompt fallback when the clipboard is blocked.
• Roman clicked "Add to Google Calendar" in his own browser: Google offered to add the right calendar.
• Independent review findings fixed (label race, https-only webcast links, $& in replacement, per-calendar link labels, copy status for screen readers, NASA endorsement line); all-day launches switching at UTC midnight kept on purpose to match the build.

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