[codex] Add minimal i18next i18n foundation (#5943)

## Thinking Path

> - Paperclip orchestrates AI-agent companies through a web control
plane.
> - The UI currently renders operator-facing copy directly from React
components.
> - Internationalization needs a smallest-possible starting point before
broader locale work can proceed.
> - The package declarations for `i18next` and `react-i18next` landed
separately, so this PR can stay focused on the implementation slice.
> - The implementation keeps the first surface English-only and
deliberately tiny while using the established `i18next` +
`react-i18next` runtime.
> - Future language contributions should be able to add a single locale
JSON file, with validation guarding key shape, interpolation parity,
suspicious payloads, and string length.
> - Locale strings must remain display-only UI copy and must not flow
into prompts, agent instructions, tool calls, shell commands, issue
content, approvals, adapter config, or other LLM-visible control paths.

## What Changed

- Initialized `i18next` behind the existing `@/i18n` boundary with fixed
English resources, fallback English, no detector plugin, no backend
plugin, no language picker, and no rich-text translation component.
- Kept `ui/src/i18n/locales/en.json` as the English source locale and
converted the validated JSON locale registry into i18next resources
before app rendering.
- Routed the no-companies start page title, description, and button
through `t(key, { defaultValue })` while preserving unchanged rendered
English copy.
- Added locale validation and focused Vitest coverage for missing/extra
keys, non-string leaves, interpolation parity, suspicious
executable/link payloads, and length caps.
- Addressed Greptile i18n review feedback: case-insensitive
event-handler detection, multi-violation diagnostics,
future-locale-friendly registration test, surfaced i18next init errors,
and removed the redundant side-effect import.
- Rebasing note: rebased onto current `public-gh/master` after the
package-only PR landed; this PR no longer changes `ui/package.json` or
`pnpm-lock.yaml`.

## Verification

- `pnpm install --no-lockfile --ignore-scripts` to install local
dependencies without reading or writing `pnpm-lock.yaml`.
- `pnpm --filter @paperclipai/ui exec vitest run
src/i18n/locale-validation.test.ts` -> passed, 7 tests.
- `pnpm --filter @paperclipai/ui typecheck` -> passed.
- `git diff --name-only public-gh/master...HEAD` shows only i18n
implementation files and the touched App copy call site; no package
manifest or lockfile changes remain in this PR.
- Visual impact is intentionally unchanged for the touched no-companies
copy because the English translations match the previous literal
strings.

## Risks

- Locale validation reduces prompt-injection risk, but the main safety
invariant is architectural: locale strings must remain display-only and
must never be used as LLM-visible control text.
- This intentionally does not add non-English locales, a language
picker, browser detection, HTTP/backend locale loading, server
localization, adapter localization, broad copy migration, or new package
scripts.
- Repository-wide CI may still depend on the separate lockfile-refresh
workflow for the already-merged package declaration, but this PR no
longer introduces package manifest or lockfile changes itself.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI Codex, GPT-5, tool-enabled coding agent in medium reasoning
mode.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] If this change affects the UI, I have included before/after
screenshots, or documented why screenshots are not applicable because
there is no intended visual change
- [x] I have updated relevant documentation to reflect my changes, or
confirmed no docs changed because behavior/commands did not change
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

ui/src/App.tsx

@@ -1,5 +1,6 @@
 import { Navigate, Outlet, Route, Routes, useLocation, useParams } from "@/lib/router";
 import { Button } from "@/components/ui/button";
+import { useTranslation } from "@/i18n";
 import { Layout } from "./components/Layout";
 import { OnboardingWizard } from "./components/OnboardingWizard";
 import { CloudAccessGate } from "./components/CloudAccessGate";
@@ -245,16 +246,21 @@ function UnprefixedBoardRedirect() {
 
 function NoCompaniesStartPage() {
   const { openOnboarding } = useDialogActions();
+  const { t } = useTranslation();
 
   return (
     <div className="mx-auto max-w-xl py-10">
       <div className="rounded-lg border border-border bg-card p-6">
-        <h1 className="text-xl font-semibold">Create your first company</h1>
+        <h1 className="text-xl font-semibold">
+          {t("app.noCompanies.title", { defaultValue: "Create your first company" })}
+        </h1>
         <p className="mt-2 text-sm text-muted-foreground">
-          Get started by creating a company.
+          {t("app.noCompanies.description", { defaultValue: "Get started by creating a company." })}
         </p>
         <div className="mt-4">
-          <Button onClick={() => openOnboarding()}>New Company</Button>
+          <Button onClick={() => openOnboarding()}>
+            {t("app.noCompanies.newCompany", { defaultValue: "New Company" })}
+          </Button>
         </div>
       </div>
     </div>