feat(i18n): localize hardcoded application copy

[opencode-agent]

Summary

• replace audited user-visible hardcoded English across App, Session UI, shared UI, and Desktop with i18n keys while preserving the existing English copy exactly
• use complete parameterized keys for composer triggers, version labels, provider labels, numbered questions, and recovery details
• localize composer placeholders, accessibility labels, menus, dialogs, toasts, Tabs help content, native Desktop menus/dialogs, picker labels, recovery copy, and app-authored WSL fallback errors
• synchronize the renderer-selected locale to native Electron surfaces with an English startup fallback
• add source-backed translations for every new key across all 27 non-English locales
• add package guidance forbidding hardcoded user-visible English, protecting designer-written English copy, and requiring CLDR, Microsoft, Apple, Mozilla/Firefox, and locale-authority sources

Scope

• localization only: key wiring, translated copy, and the minimum native translation bridge needed to render selected-locale strings
• English text and existing English keys were not changed to facilitate translations
• product/model/provider names, protocol tokens, raw external errors, code identifiers, and generated Default Project naming remain intentionally outside translation

Validation

• App i18n parity: 27 locales × 1,103 keys; no missing/extra English keys or placeholder mismatches
• UI i18n parity: 27 locales × 193 keys; no missing/extra English keys or placeholder mismatches
• App migration tests: 21 passed
• Desktop focused migration tests: 23 passed
• Session UI tests: 78 passed
• App browser tests: 41 passed
• UI tests: 27 passed
• App, Desktop, Session UI, and UI typechecks passed
• final hardcoded-copy re-audit passed for the audited production scope
• git diff --check

Requested by: @Hona (Luke Parker via Slack)