fix(dashboard): shipping method description should use rich text editor

[latifniz]

Describe the bug
The shipping method description field uses a plain textarea while the
payment method description field uses a rich text editor. Both are
customer-facing fields shown at checkout so they should behave the same way.

To Reproduce

1. Go to Settings > Shipping Methods
2. Open any shipping method or create a new one
3. Look at the Description field — it is a plain textarea
4. Compare with Settings > Payment Methods where the Description field
   has a full rich text editor

Expected behavior
The shipping method description field should use the same rich text
editor as the payment method description field.

Actual behavior
The shipping method description field uses a plain textarea with no
formatting support.

Screenshots/Videos

Environment

• @vendure/core version: 3.6.4
• Nodejs version: 22
• Database: PostgreSQL
• Operating System: Linux
• Browser: Chrome
• Package manager: npm

Additional context
Stock locations and roles also use plain textareas for their description
fields. Those are internal admin fields so a plain textarea is probably
fine there, but happy to update those too if the maintainers think it
makes sense.