fix(middleware): avoid double slash in /flashblocks redirect

[kinderjoypresents]

What changed? Why?

Fixes /flashblocks redirect path construction to avoid producing URLs with double slashes.

Notes to reviewers

Previously, redirecting from /flashblocks/* could set url.pathname to //... because subPath already begins with /.
This change normalizes the pathname before redirecting.

How has it been tested?

Manually.

Have you tested the following pages?

BaseWeb

• [] base.org
• [] base.org/names
• [] base.org/builders
• [] base.org/ecosystem
• [] base.org/name/jesse
• [] base.org/manage-names
• [] base.org/resources

[vercel]

@kinderjoypresents is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/2	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2