feat(web): support serving under a sub-path via Vite base

[voidzero]

Summary

Allow hosting the web UI under a sub-path (e.g. https://example.net/honcho/) behind a reverse proxy.

• Derive the TanStack Router basepath, the same-origin /api prefix, config.js and favicon URLs from import.meta.env.BASE_URL.
• Build with pnpm --filter @openconcho/web build --base /honcho/; the Docker image accepts --build-arg BASE_PATH=/honcho/ (the outer proxy strips the prefix, the image's nginx keeps serving at /).
• Default base stays /, so existing web, Docker, Helm and Tauri builds are unchanged.
• Docs: new "Serving under a sub-path" section in docs/docker.md.

Testing

• pnpm check (Biome, tsc, Vitest) passes; a new test asserts API_PREFIX becomes /honcho/api for a /honcho/ base.
• Image built with BASE_PATH=/honcho/: index.html references /honcho/assets/… and /honcho/config.js; /healthz, /config.js and SPA deep links return 200.
• Running behind host nginx at /honcho/ (static dist via alias, /honcho/api/ proxied to Honcho).

[offendingcommit]

Thanks, this is clean. BASE_URL is used consistently for the router basepath, the /api prefix, config.js and the favicons, and the default / leaves the web, Docker, Helm and Tauri builds as they were. The package build script is tsc -b && vite build, so --base reaches Vite as intended. CI is green.

Optional follow-up: the Vite dev middleware in vite.config.ts still mounts at the literal /api. A dev server started with a non-root --base would therefore miss the proxy. That's a dev-only edge case and doesn't block this PR.

[voidzero]

@offendingcommit Thanks for reviewing and merging this PR! I've addressed your optional follow-up about the Vite dev middleware in #112. The proxy now respects the configured base path, with HTTP regression checks for both / and /honcho/, including upstream path and query-string forwarding. Local make ci-web passed.