[Epic] Internationalize the frontend app (i18n)

[Davidnet]

Tracking issue for internationalizing the Kiji Privacy Proxy frontend (Electron renderer + main process).

Goal

Route all user-facing strings through a translation layer and ship multiple languages.

Locales

en (base) · es · fr · ja. Non-English files start as machine-drafted translations for native-speaker refinement. Japanese needs a CJK font/layout check.

Approach

• Renderer: react-i18next + i18next (+ browser-languagedetector), namespaced per feature area.
• Electron main: standalone i18next sharing the same locale JSON (separate process — menus can't use React).
• Language preference persisted via the existing Electron settings IPC.

Sub-tasks

• i18n: set up i18next infrastructure #571 — set up i18next infrastructure
• i18n: extract strings — core UI #572 — extract strings: core UI
• i18n: extract strings — settings #573 — extract strings: settings
• i18n: extract strings — dashboard & sidebar #574 — extract strings: dashboard & sidebar
• i18n: extract strings — activity & mappings #575 — extract strings: activity & mappings
• i18n: extract strings — onboarding, about & modals #576 — extract strings: onboarding, about & modals
• i18n: language selector + persistence #577 — language selector + persistence
• i18n: translate Electron main-process menus #578 — translate Electron main-process menus
• i18n: tooling, drafts & CI guardrails #579 — tooling, drafts & CI guardrails

Rollout phases

1. P1 — infra + full English extraction (no visible change; i18n: set up i18next infrastructure #571–i18n: extract strings — onboarding, about & modals #576)
2. P2 — language selector + Spanish drafted end-to-end (i18n: language selector + persistence #577, partial i18n: tooling, drafts & CI guardrails #579)
3. P3 — French + Japanese drafts, Electron menus, CJK/layout pass, CI key-parity check (i18n: translate Electron main-process menus #578, i18n: tooling, drafts & CI guardrails #579)