[FEATURE]: add locale-aware CLI and TUI strings to packages/opencode

[gqcdm]

Describe the enhancement you want to request

Add a focused multilingual MVP for packages/opencode so the main CLI/TUI can render locale-aware built-in strings.

Scope for the first slice:

• add a small config.locale setting plus environment fallback
• add lightweight en / zh dictionaries for built-in OpenCode strings
• localize selected high-frequency surfaces in the CLI and TUI such as serve, web, export, session, help/status dialogs, select defaults, and home placeholders
• keep the PR small and avoid introducing a heavy i18n framework

Benefits:

• makes the core opencode experience usable in more languages without waiting for a full translation system
• reuses the repo's existing lightweight i18n direction instead of adding a large dependency surface
• provides a reviewable first step that can be expanded incrementally later

Related context:

• broader i18n request: [FEATURE]:i18n support #7170
• larger configuration-oriented discussion: [FEATURE]:Proposal for Configuration-Based Three-Tier Language Internationalization in OpenCode #15800