chore(FR-2789): fix backend.ai-client package.json for publishable distribution

[yomybaby]

Resolves #7196(FR-2789)

Epic: FR-2792 (backend.ai-client production readiness)

Summary

Tightens packages/backend.ai-client/package.json so the package is safe to publish and predictable to consume. The package is built ESM-only by tsup (format: ['esm'] + "type": "module"), so the metadata is shaped to match an ESM-only tarball.

Changes

• exports — moved types before import per Node ESM resolution rules, no require branch (ESM-only).
• main — kept as ./dist/index.js for parity with sibling publishable packages (backend.ai-ui, backend.ai-docs-toolkit) that all retain main alongside module + exports for legacy bundlers.
• engines — added "node": ">=18" (matches the global-fetch baseline that client.ts relies on).
• publishConfig.access: public — explicit so a first publish doesn't depend on local registry defaults.
• repository — switched from a tree URL to a proper git+https:// URL with directory: "packages/backend.ai-client" so npm's "View Source" link works correctly.
• homepage — added a tree-anchored URL pointing at the package README.
• files — added README.md and LICENSE (sibling PR docs(FR-2788): add LICENSE and README to backend.ai-client #7202 / FR-2788 lands them; pack tolerates missing entries until that lands).

Out of scope

• LICENSE and README content (FR-2788 / docs(FR-2788): add LICENSE and README to backend.ai-client #7202).
• ESLint config (FR-2790 / chore(FR-2790): add ESLint flat config to backend.ai-client #7204).
• CI workflow (FR-2791 / ci(FR-2791): add backend.ai-client lint/typecheck/test/build workflow #7203).

Verification

• pnpm install from repo root — workspace link unchanged.
• pnpm --filter backend.ai-client run build — ESM 174.99 KB + DTS 65.36 KB.
• pnpm pack --dry-run (combined with docs(FR-2788): add LICENSE and README to backend.ai-client #7202) — tarball lists dist/, LICENSE, package.json, README.md.
• ESM smoke import (import { Client, ClientConfig } from 'backend.ai-client') — both resolve.
• bash scripts/verify.sh — Relay/Lint/Format pass; TypeScript pre-existing errors in client.ts exist on origin/main and are unrelated to this PR.

[yomybaby]

• chore(FR-2789): fix backend.ai-client package.json for publishable distribution #7201 👈 (View in Graphite)
• main

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[Copilot]

Pull request overview

Updates backend.ai-client’s package metadata to improve npm publishability and repo linkage within the monorepo.

Changes:

• Adjusts repository metadata to use a git URL plus directory, and adds a homepage URL.
• Updates published package contents (files) and adds publishConfig.access: public.
• Adds a Node.js engine constraint (>=18) and tweaks export field ordering.