Internationalization: string extraction, RTL, Babel, language switcher

[gregoryfoster]

Summary

Deferred i18n work beyond the minimal groundwork (lang/dir, logical properties, NFC) established in #34.

Scope

• String extraction: move all user-facing strings out of Python route handlers into Jinja2 templates
• RTL CSS rules: [dir="rtl"] overrides for sidebar, drawer, flash region positioning
• Babel/gettext integration: wrap template strings in _(), configure Babel for extraction and compilation
• Language switcher UI: dynamic lang and dir attributes, user preference persistence

Context

Minimal i18n groundwork (HTML lang/dir, <meta charset>, CSS logical properties, NFC normalization) ships with #34. This issue covers the remaining tiers needed for full multi-language support.