chore: quick wins from OSS audit (#203, #204, #211, #212)

[JonnyTran]

Summary

Bundles four low-risk quick wins from the OSS audit (filed in #203–#213). Targets the items the handover called out as "low risk, high signal."

• [ci] Fix uv cache path in extralit-server Dockerfile #203 — Dockerfile uv cache path. Moves the build-time uv cache mount in extralit-server/docker/server/Dockerfile from /root/.cache/uv to /home/extralit/.cache/uv and sets UV_CACHE_DIR explicitly so the path is consistent with the runtime user.
• [docs] Onboarding documentation cleanup pass #204 — Onboarding docs. Fixes the broken ```base fence in README.md, fills in the SDK README.md "Create an extraction schema" / "Add documents and start extraction" TBDs with a runnable Settings + Dataset snippet, points root AGENTS.md at the existing per-component CLAUDE.md files (since per-component AGENTS.md files don't exist), adds examples/README.md, and adds a "Choose a dev environment" section to CONTRIBUTING.md. The Python version claim in AGENTS.md already matches pyproject.toml (>=3.10 server / >=3.9.2 SDK), so no change there.
• [frontend] Finish Argilla → Extralit rebrand sweep #211 — Argilla → Extralit frontend rebrand. Renames BackendEnvironment.argilla → .extralit in v1/infrastructure/types/environment.ts, Environment.ts, and EnvironmentRepository.ts to match the server schema, which already returns extralit: (the previous code was silently falling back to defaults). Renames the argillaDatasets i18n key across all four locales + DatasetList.vue. Swaps docs.argilla.io → docs.extralit.ai in ja.js. Replaces the 84 Copyright 2021-present, the Recognai S.L. team headers across the frontend with the Copyright 2024-present, Extralit Labs, Inc. convention used in extralit-server. NOTICE already preserves Argilla attribution.
• [repo] Untrack editor and cache directories #212 — Untrack editor/cache dirs. Already satisfied; .cursor/, .vscode/, .ruff_cache/ are not tracked and are in .gitignore. No change.

Intentionally retained Argilla references

• how-to-configure-argilla-on-huggingface/ doc paths in nuxt.config.ts and translations — verified the renamed how-to-configure-extralit-on-huggingface/ paths 404 on docs.extralit.ai, so renaming would break links.
• argilla-dev localStorage key in extension/popup.js and useFeatureToggle.ts — would break installed dev extensions.
• argilla.imglab-cdn.net in useShareViewModel.ts — external CDN with no Extralit equivalent.
• E2E mock fixtures (argilla.span_marker_conll, argilla-other, recognai workspace) — historical test data; cosmetic.
• CHANGELOG.md [Argilla] version entries — historical.

Test plan

• cd extralit-frontend && npm run lint && npm run test — lint passed in pre-commit hook; unit tests not run locally.
• cd extralit-frontend && npm run build — Nuxt build still succeeds with the rebranded type/key.
• Smoke test: load the dataset list page and confirm extralitDatasets i18n key resolves and the BackendEnvironment.extralit settings (HF persistent storage warning, share-progress flag) light up correctly. Previously these were always falling back to defaults due to the type mismatch.
• docker build extralit-server/ — succeeds with the new uv cache path.

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