Translate welcome/onboarding & shared dialog labels (common namespace)

[meymchen]

What to build

Migrate the first-run welcome / onboarding surface and the cross-component shared dialog labels to the i18n layer from #1, with en + zh-CN translations. This slice introduces the common namespace for short shared phrases (Submit / Cancel / Back).

End-to-end behavior: a new zh-CN user sees the startup welcome message and keyboard-shortcut hints in Chinese, and shared dialog controls (Submit / Cancel / Back) are Chinese everywhere they appear.

Scope:

• Migrate the welcome/onboarding text and keyboard hints to i18n.t(...).
• Introduce locales/en/common.ts + locales/zh-CN/common.ts for shared short phrases; migrate the generic Submit/Cancel/Back-style labels across dialogs to i18n.t('common.*').
• Strings stay colored via currentTheme.

Acceptance criteria

• Welcome/onboarding text and keyboard hints come from the i18n layer.
• common namespace exists in both locales and is used by shared dialog controls.
• Locale-render assertions verify the welcome surface shows Chinese under zh-CN and English under en.
• No duplicate translations of the same shared phrase across components — they share common.* keys.

Blocked by

• i18n walking skeleton: language config + I18n layer + startup wiring + live footer #1