fix(i18n): localize hardcoded tooltips and aria-labels

[odn775]

Summary

A few user-facing strings in ChatInput and ChatMinimap were still hardcoded in English, so they rendered untranslated when the UI was set to Simplified or Traditional Chinese. This routes them through the i18n layer.

Changes

ChatInput

• ModelErrorBanner and ModelScopeWarningBanner now use useI18n() for their titles.
• Localized the title attributes on the "Input history" panel header and the Steer / Follow-up buttons.

ChatMinimap

• Localized the aria-label and title of the "Locate assistant message" jump button.

Messages
Added 7 keys to en, zh-CN, and zh-TW:

• chat.steerHint
• chat.followUpHint
• chat.inputHistory
• chat.modelError
• chat.modelScopeWarning
• chat.modelScopeWarnings
• chatMinimap.locateAssistant

Notes

• Follows the convention in docs/i18n.md that title / aria-label / placeholder values go through t().
• Updated the three banner tests in components/ChatInput.test.mjs to render inside I18nProvider, since the banners now consume the i18n context.

Testing

• npm test — all passing.