[codex] Add UI i18n runtime packages (#6058)

## Thinking Path

> - Paperclip orchestrates AI-agent companies through a web control
plane.
> - The UI i18n slice needs `i18next` and `react-i18next` available as
runtime packages before the implementation PR can stay focused on code
changes.
> - The implementation PR should not mix package declaration work with
Greptile-driven i18n code feedback.
> - This pull request isolates only the package manifest additions
requested by the maintainer.
> - The benefit is a tiny dependency-declaration PR that can be reviewed
and merged independently before rebasing the i18n implementation PR.

## What Changed

- Added `i18next` to `ui/package.json` dependencies.
- Added `react-i18next` to `ui/package.json` dependencies.
- Intentionally did not change `pnpm-lock.yaml`, matching the repository
policy that PRs do not commit lockfile changes.

## Verification

- `node -e
"JSON.parse(require('fs').readFileSync('ui/package.json','utf8'));
console.log('ui/package.json valid JSON')"`
- `git diff --name-only public-gh/master...HEAD` shows only
`ui/package.json`.
- `npm view i18next version` -> `26.2.0`.
- `npm view i18next@26.1.0 version` -> `26.1.0`.
- `npm view react-i18next version` -> `17.0.8`.
- `npm view react-i18next@17.0.7 version` -> `17.0.7`.
- Did not run `pnpm install --frozen-lockfile` because this PR
intentionally changes only `ui/package.json` and leaves lockfile
handling to the repository's separate lockfile workflow.

## Risks

- CI jobs that run `pnpm install --frozen-lockfile` may fail until the
repository lockfile workflow handles these package declarations.
- Low behavioral risk: this PR does not import or execute the packages
and changes no runtime code.

> 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 the applicable local validation for this manifest-only
change
- [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 runtime UI 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/package.json

@@ -52,12 +52,14 @@
     "clsx": "^2.1.1",
     "cmdk": "^1.1.1",
     "hermes-paperclip-adapter": "^0.2.0",
+    "i18next": "^26.1.0",
     "lexical": "0.35.0",
     "lucide-react": "^0.574.0",
     "mermaid": "^11.12.0",
     "radix-ui": "^1.4.3",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
+    "react-i18next": "^17.0.7",
     "react-markdown": "^10.1.0",
     "react-router-dom": "^7.1.5",
     "remark-gfm": "^4.0.1",