chore(i18n): add linter enforcing alphabetically sorted locale files

[gantoine]

Description
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Adds a linter that enforces the locale JSON files under frontend/src/locales/** to have their keys sorted alphabetically, so translation files stay consistent and diffs stay clean.

• New frontend/src/locales/check_i18n_sorted.py, mirroring the existing check_i18n_locales.py:
  • Walks every <locale>/<namespace>.json file and fails (non-zero exit) if keys aren't sorted alphabetically. Sorting is applied recursively, so nested objects (e.g. settings.json) are covered.
  • Supports --fix to sort files in place, emitting Prettier-compatible output (2-space indent, unicode preserved, trailing newline) so trunk check doesn't reflag them.
• Wired into CI: a new "Check locale files are sorted" step in .github/workflows/i18n.yml, alongside the existing parity check.
• Documented the new command in .claude/skills/frontend-i18n/SKILL.md and the CLAUDE.md quick command reference.
• Sorted all existing locale files (one-time reorder across all 18 locales). This is pure key ordering, no translation content changed.

Verification

• python3 frontend/src/locales/check_i18n_sorted.py → passes.
• python3 frontend/src/locales/check_i18n_locales.py → still passes (parity unaffected).
• npx prettier --check "src/locales/**/*.json" → clean.
• trunk check on changed files → no issues.

AI assistance disclosure
This PR was written with AI assistance (PostHog Code / Claude). The linter script, CI wiring, docs updates, and the one-time sort were generated and verified with an agent, then reviewed.

Checklist
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• I've tested the changes locally
• I've updated relevant comments
• I've assigned reviewers for this PR
• I've added unit tests that cover the changes

Screenshots (if applicable)

N/A (tooling / CI change)

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Created with PostHog Code

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