Add shared Raspberry Pi signage

[ScriptsSandbox]

What changed

• adds the complete Raspberry Pi signage application under signage/
• supports separate classroom and mezzanine display profiles from one codebase
• reads classroom bookings from H-Lab 1 and mezzanine reservations from H-Lab 2
• preserves Sandbox Access rules for the classroom while treating empty mezzanine calendars as available
• adds the navy and warm-cream mezzanine theme
• includes the Google Apps Script feed, NOAA and visibility integrations, offline caching, kiosk installer, profile-aware deployment, tests, and durable setup documentation

Why

The same information-design system now needs to run on two Raspberry Pis with different room calendars, availability semantics, and palettes. Keeping both displays in one maintained application prevents the installations from drifting apart.

Validation

• 21 Node tests pass
• classroom and mezzanine scenarios visually checked at 1920×1080 against the fixed 3840×2160 canvas
• remote diff verified to add or modify only files inside the new signage/ directory

Deployment note

The deployed Google Apps Script web app must be updated from signage/apps-script/Code.gs and redeployed before the mezzanine Pi can request its live calendar profile.