fix: Preserve resource URL expression formatting

[kof]

What changed

• preserve the user’s original whitespace and newlines in resource URL binding expressions

Change of plan

Automatic expression formatting is not needed. Prop binding expressions already preserve user-entered formatting; only resource URL bindings were losing their newlines. This PR aligns resource URL bindings with the existing prop binding behavior and does not add a formatter or change expression formatting rules.

Root cause

The resource URL expression was stored in an input hidden with the HTML hidden attribute. Because its input type still defaulted to text, the browser value-sanitization algorithm removed newline characters before the form was serialized.

The URL form field now uses type="hidden", which preserves the original expression during form submission.

Validation

• pnpm --filter @webstudio-is/builder typecheck
• pnpm lint
• git diff --check

Closes #5858