feat(i18n): add Spanish (es) locale translation

[lacasoft]

Summary

• Add complete Spanish (es) locale translation file (locales/es.json)
• Covers all 17 sections and 629 translation keys, matching en.json structure exactly
• languages.json already includes the Spanish locale definition, so this file is auto-detected by both the frontend (vue-i18n) and backend (locale.py) i18n systems
• No code changes required — the existing i18n infrastructure picks up new locale files automatically

Details

• Natural Latin American Spanish used throughout
• All placeholders ({error}, {count}, {name}, etc.) preserved
• Technical terms kept untranslated: MiroFish, GraphRAG, Zep, LLM, ReACT, InsightForge, PanoramaSearch, QuickSearch, InterviewSubAgent, MBTI, Twitter, Reddit
• UI symbols (✓, ✗, →, ├─, └─) preserved as-is

Test plan

• Run npm run dev and switch language to "Español" via the language switcher
• Verify all 5 workflow steps display correctly in Spanish
• Verify backend API error messages return in Spanish when Accept-Language: es header is sent
• Verify fallback to Chinese works if a key is missing