i18n: prepare user interfaces for internationalization

[TFT444]

Goal

Prepare OpenShield user-facing interfaces for localization and address the OpenSSF Silver internationalization criterion tracked in #199.

Scope

• Inventory hardcoded user-facing strings in the dashboard and website
• Select a maintained localization/message-catalog approach
• Separate interface messages from security data and rule identifiers
• Add locale-aware date, time, number, and plural formatting
• Define fallback-locale behavior
• Add tests that detect missing message keys

Completion

• Internationalization architecture and supported scope are documented
• Core dashboard navigation and common messages use catalogs
• English fallback behavior is tested
• Dates and numbers use locale-aware formatting
• Contributor translation guidance is available
• Silver evidence register is updated honestly

[TFT444]

Implementation is ready in #206. English/Spanish catalogs, fallback behavior, locale selection, Intl formatting, contributor guidance, and catalog tests are included. All CI and security checks pass.