feat: add Remote Resources: Authentication + Custom Spectral Rulesets

## Remote Resources: Authentication + Custom Spectral Rulesets

Add support for authenticated remote $refs and custom Spectral governance rules.

### Auth for remote resources
- Per-host auth headers config (Bearer, API key, Basic) stored in localStorage via settings
- Applied to all remote `fetch()` calls: `$ref` resolution and ruleset loading
- New "Remote Access" tab in Settings modal

### Custom Spectral rulesets
- New `SpectralService` runs alongside the parser and merges diagnostics into the existing Problems tab
- Rulesets configured by URL in settings (enable/disable per ruleset)
- Two loading modes:
- **`prebuilt`**: user runs `spectral bundle` locally, hosts the ESM output, provides URL — loaded via dynamic `import()`
- **`server`**: Next.js API route fetches CJS rule files (with auth), bundles via esbuild + `@stoplight/spectral-ruleset-bundler`, returns ESM to browser