feat: Configurable base path for admin/API routes

[schiste]

The /_emdash/ prefix is currently hardcoded across route injection, middleware, the admin SPA, and the CLI. Would you be open to making it configurable via EmDashConfig?

emdash({
basePath: '/_cms', // default: '/_emdash'
})

Use cases:

• Security hardening: non-default admin paths reduce drive-by scanning and brute-force surface. Not security-by-obscurity alone, but a useful layer alongside auth.
• Reverse proxy / multi-tenant setups: when multiple services share a domain, a custom prefix avoids collisions (that's kind of my use case, I try to not have tools in the main domain and would like to have the admin in a subdomain fully gated behing CF)
• Branding: some teams prefer their own path convention.

Scope assessment (from reading the source):

The change touches around 23 files but is mechanical in most. Route injection is already centralised in injectCoreRoutes() which makes the evolution not that hard to implement. The main work would be:

1. Add basePath?: string to EmDashConfig (default '/_emdash')
2. Expose it via the virtual module so middleware and admin SPA read it at runtime
3. Replace the ~313 hardcoded "/_emdash/" route strings with the config value

DB table names (_emdash_collections etc.) and internal variable names (__emdashConfig) are unaffected as only URL paths change.

It's definitely not the most important feature I know but it's a low hanging fruit :)

Thanks for the hard and work and happy to contribute the PR if the approach looks right.

Christophe