Add opt-in i18n foundation for apps (Language setting + injected helpers)

[avandeputte]

Summary

Adds an opt-in internationalization foundation so apps can render in the user's language — without changing the contract for any existing app.

The whole feature sits behind a new "Enable internationalization" master toggle, which defaults to OFF. With it off, the UI and behaviour are identical to today.

What's in it

• server/i18n.py + server/i18n_data.json — a logic/data split so translations live in data, not code. Strings are grouped by semantic topic domain (the gettext msgctxt model), which keeps homographs distinct (weather HIGH vs tides HIGH; M as minutes vs metres). ~34 languages including regional variants (pt-BR, es-MX, fr-CA…) that inherit their base language's translations. Every key carries a context note for translators.
• server/location.py + server/weather.py — one shared geocoder, babel-backed currency/country lookup (no hardcoded tables), and a 10-minute TTL cache on weather so the helper can't be called excessively.
• Runtime injection — get_plugin_pages() inspects fetch()'s signature and passes i18n / get_weather / get_location only when the app declares them. Classic 4-arg apps are completely untouched.
• Settings UI — a Language selector and a 🌐 badge, both gated by the master toggle. The toggle carries a disclaimer that it only makes sense on modules running Universal Firmware with a configurable character map (a stock reel set is ASCII-only, so it has no accented flaps).
• Gateway RX decode fix — the gateway receive path decoded/re-encoded frames as UTF-8 with errors="ignore", which silently dropped lone high bytes (0x80–0xFF). That corrupted a module's A-command character-map sync for any reel set containing extended glyphs. It now uses latin-1, a transparent byte↔code-point codec, so the raw cp1252 bytes survive to the downstream .decode('cp1252'). This is a standalone bug fix and can be cherry-picked separately if you'd prefer.

Compatibility

• The toggle defaults off → no behaviour change for existing installs.
• babel is a soft dependency; i18n.py falls back to English/strftime without it.
• Existing 4-arg apps receive exactly the call they get today.

Testing

• Full suite green (52 tests), including new tests/test_i18n.py and tests/test_gateway_transport.py.
• Verified this branch works against the unmodified upstream apps — i.e. the foundation alone is backward compatible.
• Run end-to-end on real hardware.

The apps that adopt this foundation are in a follow-up PR.

🤖 Generated with Claude Code