Add multi-language support to the UI #2

OTE Reader's UI is currently hardcoded to Spanish: every string in index.html and app.js (buttons, labels, empty states, messages, the demo/settings bars, etc.) is written in Spanish, and <html lang="es"> / <ote-events lang="es"> are both fixed.

The embedded <ote-events> widget already supports lang="en" / lang="es" internally (see its Strings table in @opentechevents/embed), so the event-card chrome is ready for this — it's the reader's own UI strings that need to become translatable.

Scope

• Extract UI strings from app.js/index.html into a translатable structure (e.g. a strings table keyed by language, similar to the embed widget's own STRINGS object).
• Add a language switcher (or respect navigator.language with a manual override, persisted like the theme preference).
• Wire the chosen language into <ote-events lang="..."> and <html lang="...">.
• Start with English as the second language, since that's what the embed widget already ships.

Out of scope

• Translating feed/event content — that's up to each community's own feed, not the reader shell.

Follow-up from the discussion in #1.