Dev setup with config file and Vite proxy

## Parent PRD

#1660

## What to build

Update the development workflow to use the new config system. `pnpm dev` works with zero configuration (no config file needed, no connections). Add `graph-explorer.config.example.json` to the repo as a starting point. Gitignore `graph-explorer.config.json`.

Update `vite.config.ts` to proxy all `/api/*` routes to the Express server (hardcoded list). Remove the old `envPrefix: "GRAPH_EXP"` filtering, `GRAPH_EXP_ENV_ROOT_FOLDER` base path logic, and the `/defaultConnection` + `/status` proxy entries.

Remove the old `.env` file from `packages/graph-explorer/` and any `dotenv` usage in the client build.

## Acceptance criteria

- [ ] `pnpm dev` starts successfully with no config file present
- [ ] `graph-explorer.config.example.json` checked into repo with documented example
- [ ] `graph-explorer.config.json` added to `.gitignore`
- [ ] Vite proxies `/api/*` to Express dev server
- [ ] Old `envPrefix`, `GRAPH_EXP_ENV_ROOT_FOLDER` base path, and `.env` loading removed from Vite config
- [ ] Old `packages/graph-explorer/.env` file removed
- [ ] Dev server works end-to-end with a config file containing a connection

## Blocked by

- Blocked by #1661
- Blocked by #1663

## User stories addressed

- User story 3
- User story 4

## Related Issues

- Part of #1660