Send block solve payloads in request bodies

[ApocDev]

Large production blocks fail live solving because solveBlockFn is declared as a GET server function. TanStack Start serializes the complete SolveInput document into the query string; Primary Mall currently produces a 29,586-character request URL and Node/Vite responds with HTTP 431.

Change the live solve server function to POST so the block document is sent in the request body, and verify a large block solves without oversized request URLs.

[ApocDev]

Fixed in commit 821d2fd. Live block solves now use POST; Primary Mall was rechecked with a 166-character request URL and a 200 response.