feat(base-path): subpath mounting via BASE_PATH env

[guimard]

Summary

Enables deploying the indexer-ui under a subpath (e.g. /indexerui/) on the same vhost as the OpenRag backend. This removes the need for separate hostnames and eliminates the cross-origin cookie problem when the browser hits both front and back under different origins.

Changes

• svelte.config.js: kit.paths.base = process.env.BASE_PATH ?? ''
• Dockerfile: ARG BASE_PATH forwarded to the build stage.
• Every internal fetch / navigation now prefixed with base from $app/paths:
  • src/lib/api/index.ts — loadConfig()
  • src/lib/components/indexer/Header.svelte
  • src/lib/components/layout/NavBar.svelte
  • src/lib/components/dashboard/Header.svelte
  • src/routes/(home)/+page.svelte
  • src/routes/+layout.svelte
  • src/routes/indexer/{+layout,+page}.svelte
  • src/routes/indexer/partition/[partition]/+page.svelte

Backward compatibility

BASE_PATH unset → empty base → root-level deployment exactly as today.

Deploying under a subpath

docker build --build-arg BASE_PATH=/indexerui -t indexer-ui:subpath .

Then the reverse proxy must forward /indexerui/* to the container without stripping the prefix (SvelteKit with paths.base expects to see the prefix). Example nginx:

location /indexerui/ {
    proxy_pass http://indexer-ui:3000;   # NO trailing slash = path kept intact
}

Test plan

• npm run check — no new errors (8 pre-existing)
• npm run build — succeeds
• Root deployment (no BASE_PATH) — full flow still works
• Subpath deployment (BASE_PATH=/indexerui) — logo, nav, partition links, loadConfig, goto("/") all resolve under the prefix