Support sub-path deployment for Docker and Vite

[gudaoxuri]

Motivation

• Private deployments frequently expose several services through a single gateway or domain, so drawDB must continue to work when served from a sub-path (context path).

Changes

• Vite (local/testing) – Document that when building or serving with Vite directly, VITE_BASE_PATH can still be set manually to mirror the desired sub-path during testing.
• Docker deployment – Bake the __BASE_PATH__ placeholder into the built assets, rely on DRAWDB_BASE_PATH at runtime to rewrite paths and generate the matching Nginx config, and update the README to reflect the simplified workflow.

Testing
Local testing passed.

[vercel]

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