Standardize tokenId type to bigint across components

[ArturDolzan]

In the staking governance page, currently, tokenId has inconsistent typing across the codebase. In some places it's string, in others it's bigint. Since the contract defines tokenId as uint256 (which maps to bigint in TypeScript), we should standardize on bigint everywhere.

Proposet solution:
API/Subgraph: Accept tokenId as string, convert to bigint immediately

[gndelia]

API/Subgraph: Accept tokenId as string, convert to bigint immediately

Note that bigint can't be serialized (to transmit it over the network), so the API has to return it as a string.. the frontend should then parse it (making a clarification just in case)