Filter on custom schemas in the schema selector

[maximized-studio]

The schema selector is used in multiple places in the Supabase UI.

Right now it mixes my custom schemas with Supabase/system schemas, which makes it cognitively noisy to use.
Personally I never need to interact with the system schemas.

It would help a lot if we could either:

• filter/hide system schemas
• or visually distinguish custom schemas from system schemas

[ysajang]

I'd like to take a stab at this!

Here's my proposed approach:

1. Identify all schema selector instances in apps/studio
2. Classify schemas into two groups:
   • System/internal: pg_catalog, information_schema, pg_toast,
     supabase_*, extensions, graphql, graphql_public, etc.
   • User/custom: everything else
3. Add a toggle or visual separator to filter/group them in the dropdown

Questions for maintainers:

• Would you prefer a toggle to hide system schemas or a
  grouped dropdown (custom schemas on top, system schemas collapsed)?
• Is there an existing utility or constant that already defines
  which schemas are "system" vs "user"?

Happy to submit a draft PR for feedback.

[ysajang]

I've submitted a PR for this: #46186

The approach groups schemas into user-defined (shown first) and system/internal (shown below a separator with muted text), reusing the existing INTERNAL_SCHEMAS constant.