feat(i18n): complete English translation + full-stack bilingual support (en/zh-CN)

[hackingbutlegal]

Summary

Complete English localization plus full-stack bilingual infrastructure — frontend UI, backend API messages, and LLM prompt templates — all in a single, atomic PR.

Why this PR over existing translation PRs

There are several open English translation PRs (#119, #155, #176, #189, #198, #244, #245, #290). This one is the most complete:

Capability	This PR	#244+#245	#189	#290	Others
Frontend vue-i18n with toggle	482 keys	262 keys	Yes	No	Partial
Backend API message catalog	84 keys	No	Parameter-based	No	No
LLM prompt template files	35 files/locale	Inline if/else	Parameter-based	No	No
Background thread locale safety	Explicit param	Yes	Unknown	N/A	No
Code comments + docstrings translated	All 57 files	No	43 files	15 files	Varies
Single atomic PR	Yes	Split across 2 PRs	Yes	Yes	Varies
Extensible to new languages	Add locale dir + JSON	Modify code	Modify code	N/A	N/A

Key architectural advantage: LLM prompts are externalized into backend/app/prompts/{en,zh-CN}/*.txt template files rather than embedded in Python code with if/else branches. Adding a new language (e.g., Japanese, Korean) means adding a directory of .txt files — zero code changes.

What's included

Layer 1: Frontend (vue-i18n@9)

• 482 translated keys per locale in frontend/src/i18n/{en,zh-CN}.json
• Language toggle in Home and MainView navbars
• Accept-Language header sent on all API requests via Axios interceptor
• fallbackLocale: 'en' for graceful degradation

Layer 2: Backend API Messages

• 84 message keys in backend/app/i18n/{en,zh-CN}.json
• msg(key, **kwargs) helper with placeholder substitution
• Flask before_request middleware sets g.locale from Accept-Language

Layer 3: LLM Prompt Templates

• 35 prompt files per locale in backend/app/prompts/{en,zh-CN}/
• load_prompt(name, locale) helper with English fallback
• Covers: ontology generation, agent profiles, simulation config, report planning/generation/chat, Zep graph queries
• Locale passed explicitly to background threads (not from Flask request context)

Translation (prerequisite)

• All Chinese text in 57 files replaced with English: UI labels, code comments, docstrings, log messages, error messages

How it works

User clicks language toggle (or browser auto-detects)
  → Frontend: vue-i18n switches locale, re-renders all $t() strings
  → Axios: sends Accept-Language header on every request
  → Flask middleware: sets g.locale from header
  → API handlers: msg('key') returns localized error/status strings
  → Background threads: locale captured before spawn, passed as parameter
  → LLM services: load_prompt('name', locale) reads locale-specific .txt files
  → LLM output: generated in the user's chosen language

Test plan

• grep for CJK characters across source files returns zero matches
• All Python files pass python -m py_compile
• Frontend builds with vite build
• Frontend keys: 482 en = 482 zh-CN (zero missing)
• Backend keys: 84 en = 84 zh-CN (zero missing)
• Prompt files: 35 en = 35 zh-CN (zero missing)
• Placeholder variable names match between en and zh-CN files

[hackingbutlegal]

Come on merge this thang