[Epic] Internationalize the frontend app (i18n)

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

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

Rollout phases

1. P1 — infra + full English extraction (no visible change; #571–#576)
2. P2 — language selector + Spanish drafted end-to-end (#577, partial #579)
3. P3 — French + Japanese drafts, Electron menus, CJK/layout pass, CI key-parity check (#578, #579)