[dasc] Improve Schema Editor UX and Schema Validation

## Summary

After extensive use of the [Schema Editor](https://da.live/apps/schema#/exp-workspace/frescopa), several usability issues have become apparent. This proposal focuses on improving the schema management experience while also strengthening schema validation to prevent unsupported or invalid schemas from being created.

## Proposed Improvements

### UI Enhancements

#### 1. Replace the schema selector with a navigation list

The current dropdown works well for a small number of schemas but becomes difficult to use as the list grows.

**Proposal:**

- Replace the dropdown with a dedicated schema list in the left of the schema editor.
- Make switching between schemas faster and more intuitive with a click.

#### 2. Add local search

Finding a specific schema becomes increasingly difficult as the number of schemas grows.

**Proposal:**

- Add a client side search field to quickly locate schemas.
- If a sidebar list is implemented, the search should simply filter the visible schemas.

#### 3. Improve the "Create Schema" workflow

Creating a new schema is currently hidden behind the schema selection dropdown.

**Proposal:**

- Provide a dedicated **Create Schema** button or other clearly visible entry point.
- Make the creation workflow more discoverable and user friendly.

#### 4. Support deep linking

It should be possible to open the Schema Editor directly with a specific schema selected.

**Proposal:**

- Support URLs that reference a schema (for example, via a route or query parameter).
- Enable bookmarking (browser) and sharing links to individual schemas for quick access.

## Hardening

### Validate schemas with `da-sc-sdk`

Leverage the `da-sc-sdk` to validate schemas before they are created or saved.

**Benefits:**

- Prevent unsupported or invalid schemas from being created.
- Ensure consistency with the SDK validation rules.
- Reduce runtime errors caused by malformed schemas.

Start with the Schema Editor linked in the issue and trace how schemas are selected, created, routed, and saved. Review the da-sc-sdk validation entry points before deciding where validation belongs. Done means the editor provides list navigation, local search, a visible creation action, deep links to schemas, and rejects unsupported or invalid schemas.

Written by the indexing model from the issue text.