feat(control-plane): add i18n framework

[XIYBHK]

Summary

• Add an i18next/react-i18next localization framework for the control plane.
• Add English and Simplified Chinese resource files plus locale resolution and persistence.
• Add a language selector and initial localized coverage for the app shell, dashboard, login, bank overview, recall, reflect, and entities views.

Notes

• This is an initial framework and coverage pass, not a complete translation of every control-plane string.
• Developer/API terms such as GitHub, JSON, API, IDs, event names, and raw data values are intentionally preserved where appropriate.
• Locale preferences are applied after hydration to avoid server/client text mismatches.

Closes #1660.

Verification

• sc --noEmit in hindsight-control-plane
• Targeted ESLint check for changed i18n/control-plane files (0 errors; existing warnings remain in touched legacy views)

ext build in hindsight-control-plane

• Browser check for Chinese rendering on login, recall, and reflect with 0 new hydration errors

[XIYBHK]

Update: pushed a follow-up hydration fix. The root layout now resolves the initial locale from the locale cookie or Accept-Language, passes it to html lang and I18nProvider, and the provider creates a per-instance i18next setup instead of doing a delayed client-side switch. Verified tsc --noEmit, targeted ESLint, next build, and browser reloads in zh-CN/en/zh-CN with 0 fresh hydration mismatch logs.

[MapleEve]

Working on it

[MapleEve]

I pushed a stacked PR against XIYBHK:control-plane-i18n so this can be merged into this draft PR instead of competing with it: XIYBHK#1

Scope covered there:

• expanded control-plane i18n key coverage across UI/components and API errors
• added zh-TW (正體中文（臺灣）)
• added canonical yue-Hant (粵語（繁體）) for general/world Cantonese in Traditional script, with yue-Hant-HK/yue-HK/yue aliases kept for compatibility
• localized API error response text through i18n resources via errorKey, instead of hardcoding Chinese responses
• kept all locale schemas aligned: en, zh-CN, zh-TW, and yue-Hant each have 1011 leaf keys; missing/extra keys are 0
• covered 78 API errorKey values in all four locales

Validation note: git diff --check and the targeted i18n coverage checks passed. tsc --noEmit still reports existing non-i18n issues around the observations route implicit any and missing @vectorize-io/hindsight-client type resolution.

[nicoloboschi]

superseded by #1775

feel free to add/improve more languages @XIYBHK @MapleEve , thanks

[MapleEve]

Update: #1729 is now superseded by the official i18n PR #1775, which has been merged into main.

I rebased/ported the Chinese locale follow-up onto the official next-intl message catalog structure and opened a new upstream PR here: #1784

That PR adds zh-CN, zh-TW (正體中文（臺灣）), and canonical yue-Hant (粵語（繁體）) on top of the official 8-locale implementation, and also localizes API error payloads through message keys without exposing errorKey in the response body.