feat(i18n): add Spanish (es) locale — 15 namespaces, ~1 450 keys

[Mukller]

Summary

Closes #197 — adds a complete Spanish (es) locale to the i18n layer.

What's included

• 15 JSON namespace files in client/src/i18n/locales/es/:
  activity, alerts, analytics, ccConfig, common, dashboard,
  errors, kanban, nav, run, sessions, settings, splash,
  updates, workflows
• ~1 450 translated string keys — full parity with the en locale
• Upstream-only keys included: awaitingReason (common), waitingBanner /
  conversation / timeline / transcriptNotFound (sessions),
  remoteSources / cursorPathsNote (settings), clearSearch / jumpTo /
  new keybinding fields (ccConfig)
• Language switcher wired: languageNames.es = "Español" and
  languageShort.es = "ES" added to nav.json for all five locales
• client/src/i18n/index.ts: imports all 15 es bundles and adds "es"
  to supportedLngs
• client/src/i18n/__tests__/i18n.test.ts: three new assertions —
  Spanish nav keys, non-explicit es-ES tag resolution, and terminology check

Translation notes

• Interpolation variables ({{count}}, {{name}}, <code>…</code>) are
  preserved verbatim throughout
• The word Agent (as in the Claude agent concept) is rendered as agente /
  subagente — the natural Spanish equivalent; technical UI identifiers such
  as toolLegend.agent remain untranslated, matching the project convention
• All 16 splash taglines and 11 chart-info what/howToRead/why blocks
  are fully translated

Test plan

• Switch language to Español in the dashboard Settings → language picker
• Verify nav labels, dashboard cards, session detail, Kanban, and Workflows
  page all render in Spanish
• Verify es-ES browser locale also resolves to es (nonExplicitSupportedLngs)
• cd client && npx vitest run src/i18n — three new Spanish test cases pass

[Dcastroro]

The new Spanish option is only added to the Spanish nav file. The English, Chinese, Vietnamese, and Korean nav resources do not contain languageNames.es or languageShort.es. Since the language selector renders while one of those locales is active, users cannot get the localized Spanish label before switching and will see the raw key or fallback behavior. Add the Spanish language name and short label to every existing nav locale, and extend the locale parity test so future languages must be discoverable from every active locale.

[hoangsonww]

Thanks for the changes -- mostly lgtm, but can you also add Spanish support to the documentation (README in spanish, the wiki page at wiki/index.html, etc.)? If you're using AI agents, it can be as easy as telling the agent to do the docs update skill already in the repo.