chore: remove hardcoded Vite proxy IP

[DeepZone]

Motivation

• The frontend Vite dev proxy contained a hardcoded private IP (http://192.168.58.167:8000) which breaks portability for self-hosters and must not be committed.
• Make the proxy target configurable via environment variables so different deployments can use their own hostnames/IPs without code changes.

Description

• Replaced the hardcoded proxy in frontend/vite.config.ts with an environment-driven value const apiProxyTarget = process.env.VITE_API_PROXY_TARGET || 'http://backend:8000' and set target: apiProxyTarget.
• Added VITE_API_PROXY_TARGET=http://backend:8000 to .env.example to document the frontend Vite proxy default.
• Updated README.md selfhosting/networking guidance with Docker Compose and local development defaults and an explicit note to avoid committing local/private IPs.
• Searched the repository for private IP patterns (192.168.*, 10.*, 172.16-31.*) and removed the single tracked occurrence from the frontend config.

Testing

• Built the frontend with cd frontend && npm run build, which completed successfully and produced the production build.
• Ran backend tests with cd backend && pytest -q, and the test suite passed (102 passed).

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Codex Task